W. G. VanNamd — Embryology of Eustylochus. 293 



pole, and finally encloses the whole Qgg, the large cells of the lower 

 hemisphere becoming enclosed within the ectoderm layer. In this 

 manner a spherical embryo is formed which is really a modified 

 gastrula. 



The time occupied in developing up to this stage is variable. Eggs 

 may occupy eight hours or more between the two and the thirty- 

 two cell stage. In the course of a few hours more the cells become 

 so numerous that it is difficult to count them. During the two or 

 three days following the time when the ectoderm overgrows the egg 

 the interior cells undergo a change, their outlines become less dis- 

 tinct than those of the ectoderm cells, and nothing but the nuclei 

 and yolk spheres can be distinguished in the living specimen. The 

 consistency of the interior cells becomes less firm and they seem to 

 be more or less free to move about. 



As the development proceeds a number of large oval bodies of 

 different sizes become conspicuous within the embryo. These are 

 shown in the figures of the later stages (Figs. 32, 33 and 34). They 

 were noticed by Girard, who considered them to be cavities. They 

 are, however, solid bodies, — largo cells from the lower hemisphere 

 of the iigg densely packed with yolk, which seems in some of them 

 to fuse to a single large mass occupying most of the cell, like the 

 fat in adipose tissue. 



When foiir or five days ol(J the embryo, still practically spherical, 

 develops cilia, and begins to revolve within the egg-membrane. 

 About the same time active contractile movements commence. As 

 development proceeds it becomes somewhat elongated, and therefore 

 is necessarily more or less bent or folded within the membrane. By 

 this time the anterior and posterior ends may be readily distin- 

 guished, whereas in the earlier stages this is a matter of great 

 difficulty on account of the small difference in the size of the 

 blastomeres. There is, however, no reason to doubt that this is 

 already determined at a very early stage, as it is in polyclads like 

 JDiscocelis, where the conditions for demonstrating it are not so un- 

 favorable. As the embryo lies in the egg the dorsal surface is the 

 more convex; the ventral surface shows a transverse fold due to the 

 bending of the embryo already mentioned. On the posterior ven- 

 tral surface two longitudinal lobes appear, separated by a median 

 groove or depression (Fig. 32). 



About six days after the Qgg is laid a very dark brown pigment 

 spot, or eye, becomes visible near the anterior end on the left side. 

 It is at first small and irregular in shape, though it afterwards in- 



Trans. Conn. Acad., Vol. X. August, 1899. 



20 



