160 



INVEETEBEATA 



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 JVephelis, one of the Gnathobdellidae, 



as type, because the 



A hjy /m embryology of this form 



r „-=-..-.. j^g^g \,QQT^ worked out in 



recent times by Sukat- 

 schoff (1900, 1903). 

 The egg divides as 

 usual into four macro- 

 meres and these bud off 

 a first quartette of 

 micromeres. 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 

 somatoblast, 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 



show that these 

 are the missing 



will 

 cells 



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 2D. 

 From these two cells, 

 fou.r cells of micromeric 

 dimensions are formed 

 at the vegetative pole 

 of the egg. 



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

 budded off, which divides into 2d^^ and 2d2^, whilst from 2D, 3d is 

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

 into 3di and 3d^. Whether this interpretation is justified or not, we 



Fig. 119. — Two lougitudinal sections of embryos of 

 Nephelis vulgaris. {After Sukatschoff. ) 



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 ; ft,.6, head blastema; i.&, trunk blastema; 

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

 stomodaeum. 



