THEORY OP THE EMBRYONIC STAGE OF ONTOGENY. 333 



similar comparison between the segmentation of the eggs^ of 

 vertebrates, subsequently put forward the thesis,^ based on a 

 comprehensive survey of the facts of embryology, that the 

 rapidity of segmentation of a given region of the ovum is in- 

 versely propoi'tional to the amount of yolk contained in it. 



The general effect of the presence of yolk, therefore, when 

 massed specially in the endodermic end of the ovum, is to 

 impede cell division, and render processes of development 

 which depend on folding (e. g. invagination) impossible. 



There is, however, another manner in which yolk can be 

 accumulated, and that is in the more central portion of the 

 ovum, instead of at one end. This is characteristic of the 

 Arthropoda. When it is comparatively moderate in quantity, 

 as in the case of Lucifer, segmentation and invagination can 

 proceed normally, though the number of cells composing the 

 blastosphere is small. When it is somewhat greater in quantity, 

 as in Branchipus, segmentation at first proceeds normally, but 

 soon the inner yolky ends of the blastomeres fail to be governed 

 by the rapidly increasing nuclei, and segmentation only aflPects 

 the outer layer of the egg, the inner ends of the first formed 

 blastomeres fusing together to form a central yolky mass. In 

 most Crustacea the yolk is so large in quantity that only 

 superficial segmentation is possible from the beginning. In- 

 vagination of this outer layer toform the gut still occurs in some 

 cases, the yolky mass being pushed before it ; but, since the yolk 

 is eventually absorbed by the endodermic cells, even this soon 

 ceases to be possible, and we reach eventually a condition in 

 which the segmentation and first processes of development recall 

 to a certain extent those found in telolecithal eggs when the yolk 

 increases to such an extent as to prevent segmentation at the 

 endodermic pole at all (meroblastic eggs). In the scorpions and 

 insects segmentation in its earlier stages is totally suppressed 

 and represented merely by the multiplication of nuclei; and in 

 the later stages segmentation only occurs where developino- 

 organs require it, and thus a mimetic meroblastic segmenta- 

 tion is produced. 



' 'Quart. Journ.Micr.Sci.,' 1875. ^ ' Comp. Emb.,' vol. i, p. 121. 



