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sizes (Fig. 7, D). These coalesce, round up into a spherical cell (Fig. 7, E, F), which soon develops an enveloping cellulose wall, and passes as a "zygote " into a resting stage (Fig. 7, G). In this condition the organism

[graphic]

FIG. 7.-Development of Pandorina morum: A, a swarming family; B, a similar family divided into sixteen daughter families; C, a sexual family, the individual cells of which are escaping from the common gelatinous investment; D, E, conjugation of pairs of swarm spores; F, a young zygote; G, a mature zygote; H, transformation of the contents of a zygote into a large swarm cell; I, the same, free; J, a young family developed from the latter; K, a free swimming swarm spore. (After Pringsheim.)

Gradual differentiation of reductive cells.

matic cells carry on their various functions for a time, grow old, die, and disappear, certain of the germ cells. alone surviving in the production of new individuals. On the borderland between the unicellular and the multicellular organisms, however, stand certain colonial forms, which show an exquisitely graded series of steps, from the conditions of unicellular multiplication to those of the multicellular forms. Let us examine a few examples of these. Pandorina morum is a minute fresh-water Alga, consisting of a colony of sixteen ovoid cells imbedded in a spherical mass of a jelly-like substance. From each of these cells two long, hairlike flagellæ extend out freely into the water, and by their lashing to and fro the colony is propelled from place to place (Fig. 7, A). In multiplication by simple division each one of these cells divides into a group of sixteen daughter cells, the general gelatinous intercellular substance of the parent colony dissolves, the sixteen daughter colonies become free, and by continuous growth soon attain the size of the parent colony (Fig. 7, B). After a certain number of generations produced in this manner, the necessity for reproduction by conjugation ensues. In this method the sixteen cells of a colony divide, each one usually into eight minute cells, which are set free in the water by the dissolution of the common gelatinous envelope (Fig. 7, C). Each one of these swarm spores, or "zoospores," consists of an oval, greenish cell, the pointed end of which is hyaline and bears two long cilia, by means of which the spore swims through the water (Fig. 7, K). These zoospores are not all of exactly the same size, but no great difference is noticeable. If the zoospores from two different colonies come near each other, they unite in pairs made up of individuals of the same or of different

sizes (Fig. 7, D). These coalesce, round up into a spherical cell (Fig. 7, E, F), which soon develops an enveloping cellulose wall, and passes as a "zygote" into a resting stage (Fig. 7, G). In this condition the organism.

[graphic]

FIG. 7.-Development of Pandorina morum: A, a swarming family: B, a similar family divided into sixteen daughter families; C, a sexual family, the individual cells of which are escaping from the common gelatinous investment; D, E, conjugation of pairs of swarm spores; F, a young zygote; G, a mature zygote; H, transformation of the contents of a zygote into a large swarm cell; I, the same, free; J, a young family developed from the latter; K, a free swimming swarm spore, (After Pringsheim.)

Gradual differentiation of reductive cells.

matic cells carry on their various functions for a time, grow old, die, and disappear, certain of the germ cells. alone surviving in the production of new individuals. On the borderland between the unicellular and the multicellular organisms, however, stand certain colonial forms, which show an exquisitely graded series of steps, from the conditions of unicellular multiplication to those of the multicellular forms. Let us examine a few examples of these. Pandorina morum is a minute fresh-water Alga, consisting of a colony of sixteen ovoid cells imbedded in a spherical mass of a jelly-like substance. From each of these cells two long, hairlike flagellæ extend out freely into the water, and by their lashing to and fro the colony is propelled from place to place (Fig. 7, 4). In multiplication by simple division each one of these cells divides into a group of sixteen daughter cells, the general gelatinous intercellular substance of the parent colony dissolves, the sixteen daughter colonies become free, and by continuous growth soon attain the size of the parent colony (Fig. 7, B). After a certain number of generations produced in this manner, the necessity for reproduction by conjugation ensues. In this method the sixteen cells of a colony divide, each one usually into eight minute cells, which are set free in the water by the dissolution of the common gelatinous envelope (Fig. 7, C). Each one of these swarm spores, or "zoospores," consists of an oval, greenish cell, the pointed end of which is hyaline and bears two long cilia, by means of which the spore swims through the water (Fig. 7, K). These zoospores are not all of exactly the same size, but no great difference is noticeable. If the zoospores from two different colonies come near each other, they unite in pairs made up of individuals of the same or of different

sizes (Fig. 7, D). These coalesce, round up into a spherical cell (Fig. 7, E, F), which soon develops an enveloping cellulose wall, and passes as a "zygote " into a resting stage (Fig. 7, G). In this condition the organism.

[graphic]

FIG. 7.-Development of Pandorina morum: A, a swarming_family: B, a similar family divided into sixteen daughter families; C, a sexual family, the individual cells of which are escaping from the common gelatinous investment; D, E, conjugation of pairs of swarm spores; F, a young zygote; G, a mature zygote; H, transformation of the contents of a zygote into a large swarm cell; I, the same, free; J, a young family developed from the latter; K, a free swimming swarm spore. (After Pringsheim.)

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