38 TEXT-BOOK OF EMBRYOLOGY. 



rounded so that even at the four-cell stage there is a small central cavity (Fig. 

 20, A). As cleavage progresses the cells become more closely arranged and 

 pushed away from the central cavity (Fig. 20, D, E, F). At the i28-cell 

 stage all the cells are arranged in a simple epithelial layer around a rela- 

 tively large central cavity, the segmentation cavity or Uastoccel. The entire 

 structure is now the bias tula. Other divisions occur until the blastula con- 

 tains about 256 cells. There is a gradual transition from the micromeres at 

 one pole of the hollow sphere to the macromeres at the opposite pole. It 

 should be recalled here that, on account of the position of the yolk-free por- 

 tion of the ovum, the micromeres lie where the anterior region of the embry- 

 onic body will arise and the macromeres where the posterior region will 

 develop. About four hours elapse between the time the first cleavage occurs 

 and the time the 256-cell blastula is formed. 



Gastrulation. This process comprises the conversion of the single 

 walled blastula into the double walled gastrula. The vegetative pole 

 becomes flattened, the macromeres assuming columnar form. The cells at 

 the dorsal margin of the flattened pole begin to proliferate more rapidly 

 than elsewhere, as shown by the increased number of mi to tic figures (Fig. 21, 

 A, B). This area of accelerated division then extends in both directions 

 around the margin of the flat pole, forming the germ ring. Beginning at the 

 dorsal margin the macromeres are folded, or invaginated, into the blastocoel 

 until the blastoccel is obliterated (Fig. 21, C, D, E, F, G). A rough analogy 

 is the pushing in of one side of a hollow rubber ball. The invagination, how- 

 ever, is more rapid along the dorsal margin of the plate of macromeres, and 

 as the infolding progresses there is formed a plate of small cells which arise 

 through the more rapid proliferation in the germ ring (Fig. 21, D, E). On 

 the ventral side the ingrowth is but slight, the whole plate of macromeres 

 behaving as if hinged at this point. By these processes the blastula, with a 

 single layer of cells, has been converted into the gastrula, with a double 

 layer of cells and a new cavity which opens to the exterior. 



The outer layer of cells is the ectoderm which is in direct contact with 

 the environment of the developing organism. The inner layer is the ento- 

 derm which forms the lining of the new cavity, or archenteron, in the interior 

 of the organism. The entoderm consists of two types of cells, the larger 

 cells with considerable yolk content which lie on the ventral side or in the 

 floor of the archenteron and the smaller cells forming the dorsal lining of the 

 archenteron which were produced by the rapid divisions in the germ ring. 

 This latter group in part really had a brief existence as ectodermal cells and 

 then contributed to entoderm by being inflected round the rim of the opening 

 between the archenteron and the exterior. The inflection of the cells in 

 question, often called involution is therefore one of the factors in gastrula- 



