PHYLLOPODA. 123 



B. Phyllopoda. 



The formation of the germ -layers in the Phyllopoda is best known 

 in the case of the summer eggs of Moina, one of the Cladocera, 

 which have been closely investigated by Grobben (No. 11). There 

 is considerable resemblance between the processes here and those 

 already described in connection with Cetochilus. We must not, 

 however, lose sight of the fact that two factors in the development 

 of the eggs of Moina have brought about a difference : (1) The 

 nourishment by means of the blood-plasma transuding into the 

 brood-cavity, which probably leads to secondary diminution of 

 the food-yolk (in Cetochilus also the yolk seems to be secondarily 

 diminished, although from other causes), and (2) the paedopartheno- 

 genesis, which is connected with the precocious development of a 

 distinct genital cell. 



Cleavage here, as in most Cladocera, is purely superficial (Type III., 

 rf. p. 113). As early as the thirty-two-celled stage we find the 

 blastomeres at the surface fairly sharply marked off from the central 

 mass of food-yolk. As in Cetochilus, at the vegetative pole of the 

 egg, certain differentiations appear which accompany the formation 

 of the germ-layers. In this region are found those rudiments which, 

 at a later stage (after a certain amount of displacement), lie at the 

 ventral surface of the embryo. There is here a central richly- 

 granular cell, which may be called the genital cell (Fig. 58, g), and 

 from which, later, the paired genital rudiment arises. Behind this 

 lies a cell represented in the act of division, which as the entoderm- 

 cell (en) represents the rudiment of the whole entoderm. At a 

 somewhat later stage, these two rudiments have, by division, become 

 multicellular. An area composed of numerous entoderm -cells 

 (Fig. 58 B, en) can then be distinguished, and, anteriorly to this, 

 four genital cells (g). Around the latter are a number of cells 

 representing the rudiment of the mesoderm (ms). All the remaining 

 cells now form the ectoderm. 



Even at this stage the rudiment of the mesoderm has a tendency 

 to migrate inwards below the genital cells (Fig. 58 B). In later 

 stages, this process is completed (Fig. 58 0, ms). The mesoderm 

 now lies entirely within the embryo, and at the same time the 

 entodermal area becomes invaginated, the gastrula stage being thus 

 reached (Fig. 58 C). 



Soon after the mouth of the gastrula has completely closed, the 

 eight genital cells, produced by division from the four above- 



