220 



GENERAL EMBRYOLOGY 



to 2, 4, 8, 16, etc., is very rarely continued after eight or sixteen 

 cells have been formed; the regularity of division may be dis- 

 turbed as early as the second cleavage. In some forms (e.g., 

 Echinoderms, Godlewski) the nuclei of the daughter cells 

 enlarge considerably after each division, in some cases perhaps 

 even to the original size: the cell bodies fail to do so. The 

 result is the constant increase in the relative size of the cell 

 nuclei; in other words, while the amount of cytoplasm increases 

 only slightly or not at all during cleavage, the amount of nucleo- 



c 



FIG. 105. Types of blastulae. A. Amphioxus (coeloblastula). B. Petro- 

 myzon. After von Kupffer. C. Noturus (Teleost) (discoblastula). D. Clava 

 (Hydroid). After Hargitt. (Solid type.) a, animal pole; c, segmentation 

 cavity or blastoccel ; p, periblast (a non-cellular protoplasmic layer resting upon 

 the yolk mass) ; v, vegetal pole. 



plasm increases considerably, so that at the close of this period 

 the organism contains an appreciably greater proportion of 

 nuclear material than did the zygote. This, however, may not 

 be regarded as a general characteristic of the cleavage mitoses 

 in all organisms (Conklin). 



After a number of cells, varying in different species, have been 

 formed they become arranged so as to limit an internal cavity 

 filled with a fluid. In its simplest and apparently most primi- 



