No. I.] POLYCHOERUS CAUDATUS. 169 



To understand the fate of A, A' it is necessary to return to 

 the description of the ovum in its earlier stages. When the 

 ovum consists of but four cells a distinct segmentation cavity 

 exists, as is shown in section, Fig. 23. As the ovum returns to 

 its resting stage this cavity becomes obliterated by the round- 

 ing of the outer surfaces and consequent sinking in of the cells 

 (Fig. 24). This disappearance of the cavity is, however, merely 

 temporary, for with renewed karyokinetic activity of the cells 

 it is again formed. In the ten-celled stage (Fig. 8) the cavity 

 reappears again in section (Fig. 25), but to be obliterated as 

 before by the sinking in of the cells A, A', and also by the 

 change of form of all the other surrounding cells (Fig. 27), 

 particularly E, E'. At this stage the ovum becomes much 

 elongated and quite deeply arched (Fig. 10), the cells A, A' 

 being always on the concave side. 



Fig. 28 shows a horizontal section through the sixteen-celled 

 stage in which no trace of the cavity is to be found, while in 

 the next figure (Fig. 29), a cross section shows a large cavity. 

 This alternate formation and obliteration of the cavity con- 

 tinues until the ovum has obtained the magnitude of upward of 

 sixty-six cells, when the cells A, A' sink into and completely fill 

 the cavity, while the cells ^3, B' ^ and ^3, E' ^ (Figs. 16 and 18) 

 draw together and cover the space in the surface left by A, A'. 

 Sections of later stages show no trace whatever of this segmen- 

 tation cavity. On the contrary. Fig. 33, a cross section 

 through a distinctly later stage, shows an outer layer of small 

 cells, while within are a mass of much larger cells evidently 

 derived from A, A'. These inner cells form the mesentoderm 

 of the adult, while the outer ones, all descendants of B, C, D, 

 E, and B', C, B', E', form the ectoderm. At a still later 

 stage (Fig. 34) the cells of both mesendoderm and ectoderm 

 are all of about the same size, showing that the division of the 

 cells A, A' and their descendants must have been quite rapid. 

 The ectoderm is already quite diflferentiated from the central 

 mass. Soon, however, it becomes distinctly differentiated into 

 a two-celled layer, and the central mesentodermic mass seems 

 to undergo a process of degeneration. Open spaces occur, and 

 in the center the cells are less numerous, as is shown in 



