454 



ZOOLOGY FOR MEDICAL STUDENTS 



CHAP. 



through it fine granules of yolk while at increasing distances from the 

 apical pole the yolk becomes more coarsely granular and also more 

 abundant, the intervening cytoplasm becoming more and more sparse. 

 The delaying effect of the preponderating amount of yolk in the abapical 

 portion of the egg finds expression in a prolonging of the intervals between 

 successive divisions. The result is that the blastomeres of the abapical 



>n of the egg segment more slowly and a're therefore of markedly 

 lar-er size than those of the apical region. The segmentation of the egg 

 in such a case is said to be unequal. 



In the case of Amia (Fig. 190, B) the proportion of yolk in the 



ubupiral portion of the egg preponderates still more, and the rapidity of 



H 'ntation is still more delayed, the meridional furrows that make 





B C 



FIG. 190. 



Ganoid eggs in course of segmentation. A, Acipenser ; B, Amia ; C, Lepidosteus. 

 (After Bashford Dean, Kycleshymer, and Whitman.) 



their appearance at the apical pole spreading downwards with extreme 

 sluggishness. 



Finally in Lepidosteus (Fig. 190, C) the proportion of cytoplasm to 

 yolk in the abapical hemisphere is so reduced as to render segmentation 

 impossible. T1 K - furrows spreading downwards from the apical pole 

 never extend much beyond the equator, and all the abapical portion of 

 the egg remains unsegmented a mass of yolk with only the sparsest 

 mnnants of cytoplasm between its granules. 



In Lepidosteus therefore the process of segmentation is incomplete, 

 or meroblastic as it is termed technically, in contradistinction to the 

 complete or holoblastic segmentation exemplified by the other types that 

 ha\c been mentioned. 



Tin- pro. ess of segmentation results in a stage corresponding to the 



l)I;lstnl lin/)/iioxns but peculiarities in the segmentation process 



""in peculiarities in the blastula and these in turn bring about 



peculiarities in the process of gastrulation. In the egg with markedly 



unequal segmentation the part corresponding to the large-celled portion 



