160 



INVEKTEBKATA 



CHAP. 



we might be inclined to doubt if we were acquainted only with the 

 adult structure, and secondly, because it was on the embryos of 

 Leeches that Whitman made the first studies of " cell lineage." 



We take the development of Nephelis, one of the Gnathobdellidae, 



as type, because the 



A kh 



stom 



end 



mac 



B 



m 



stom 



embryology of this form 

 has been worked out in 

 recent times by Sukat- 

 schoff (1900, 190:3). 

 The egg divides as 

 usual into four macro- 

 meres and these bud off 

 a first quartette of 

 niicromeres. These 

 micromeres increase by 

 division and form the 

 head blastema. Of the 

 second quartette of 

 micromeres apparently 

 only one member is 

 formed, viz. 2d, the first 

 sornatoblast, though the 

 statement that cells are 

 budded inwards from 

 the first quartette of 

 micromeres to surround 

 a stomodaeum or lar- 

 val oesophagus looks 

 doubtful ; possibly re- 

 newed investigation 

 will show that these 

 cells are the missing 

 members of the second 

 quartette, and are 

 budded from the macro- 

 meres directly. 2d is 

 a large cell quite equal 

 in size to its sister, the 

 residual macromere 21). 

 From these two cells, 

 four cells of micromeric 

 dimensions are formed 

 at the vegetative pole 

 of the egg. 



Probably we may interpret this statement thus : from 2d, 2d 2 is 

 budded off, which divides into 2d 21 and 2d 22 , whilst from 21), 3d is 

 budded off, sole representative of the third quartette, and this divides 

 into 3d 1 and 3d 2 . Whether this interpretation is justified or not, we 



mac. 



FIG. 119. Two longitudinal sections of embryos of 

 Nephelis vidgaris. (After Sukatschoft'. ) 



A, optical longitudinal (frontal) section of late embryo ; B, 

 optical longitudinal (sagittal) section of larva just after escape 

 from the cocoon ; end, cells (derived chiefly from 3D) forming 

 definitive endoderm ; h.b, head blastema; t.b, trunk blastema; 

 m, mouth ; mac, degenerating macromeres ; st, stomach ; stom, 

 stomodaeum. 



