144 c - M - CHILD. 



In many proglottids a certain proportion of the cleaving eggs 

 undergoes degeneration sooner or later, and such eggs usually 

 show irregular nuclear fragmentations. It is therefore possible 

 that the amitotic first cleavage may be an indication of degenera- 

 tion. Possibly such eggs are not fertilized and are therefore 

 incapable of normal development. 



But although the first cleavage is usually or always mitotic, 

 there can be no doubt that amitotic division appears very early 

 in the course of cleavage. 



In most cases a number of nuclear divisions occur before cell- 

 boundaries become visible in the egg (Fig. 21, PI. V.). Cases 

 of mitosis are rarely seen after the first cleavage but amitosis is 

 of frequent occurrence (Figs. 21-26, PI. V.). It was impossible 

 to determine whether the cleavage exhibited any regularity, for 

 no basis for orientation was discovered. As cleavage proceeds, 

 the egg is gradually divided into blastomeres containing yolk and 

 blastomeres without yolk. In earlier stages the yolk-bearing 

 blastomeres often contain two or more nuclei (Figs. 23, 26, PI. 

 V.), but in later stages after cytoplasmic cleavage is more ad- 

 vanced they usually contain one relatively large nucleus (Figs. 

 28, 29, 30, PI. VI.). In other words as these yolk-bearing blas- 

 tomeres are gradually reduced in size by successive cleavages 

 the cytoplasmic cleavages keep pace more nearly with the nuclear 

 divisions. 



In the yolkless portions of the egg, however, nuclear division 

 continues to be far in advance of cytoplasmic division as far as 

 the cleavage has been followed (Figs. 27, 29, 30, 31, PI. VI. ; 32, 

 PI. VII.) : the consequence is that each blastornere contains sev- 

 eral or many nuclei of relatively small size. Evidently the nuclei 

 in these yolkless portions of the egg are dividing much more 

 rapidly than those in the yolk-bearing portions and, as is evident 

 from the figures of Plates V. and VI., amitosis is the typical 

 method of division. 



Rarely a case of mitosis is observed : in all the hundreds of 

 eggs in cleavage stages which have been examined, not more 

 than a dozen cases of mitosis have been seen in stages later than 

 the first cleavage. When mitosis occurs it apparently always 

 involves one of the larger nuclei. Mitotic divisions of the small 



