ORGANIZATION AND CELL-LINEAGE OF A.SCIDIAN EGG. 27 



(1) The (list cleavage spindle is eccentric toward the posterior pule of the egg, 

 and the median plane of the future embryo is marked out by the bilateral symmetry 

 of the unsegmented egg (p. ti). 



(2) The plane of the lirst cleavage coincides with the plane of bilateral sym- 

 metry, and therefore divides the egg into right and left halves (p. G). 



(3) The second cleavage plane is transverse to the long axis of the embryo 

 and separates two large anterior cells from two small posterior ones (p. 7). 



(4) The intersection of these two planes marks the vertical axis of the egg; one 

 end of this axis corresponds to the middle of the dorsal, the other to the middle of 

 the ventral face of the gastrula (p. 7). 



(5) The third cleavage separates 4 larger dorsal cells from 4 smaller ventral 



a 



ones (p. i); the latter are ectodermal, the former ''mixed." 



(ti) At the fourth cleavage these 8 cells give rise to l(i; X ventral cells, all 

 ectodermal, and S dorsal cells, 6 of which are mixed, and 2, which are smaller than 

 any of the others and lie at the posterior pole, ectodermal (p. 8). 



(7) By division these 1 li cells give rise to 32; l(i ventral cells, all ectodermal 

 and Id dorsal cells. 4 ectodermal derived from the 2 posterior ectoderm cells of the 

 previous stage. 6 ectodermal derived from the 6 mixed cells. 4 endodermal and 2 

 still mixed. With regard to the identification of the dorsal and ventral faces at 

 this stage they say: "'On bien les cellules ectodermique torment ensemble une 

 calotte appliquee par sa concavite" contra les globes endodermiques et mixtes (comme 

 dans fig. 10, c), ou bien e'est le contraire qui a lieu, les globes endodermiques et 

 mixtes s'etalent en surface de facon a constituer ensemble une calotte moulee sur 

 l'ectoderme (fig, '*. c)." 



(X) At the next stage there are 44 cells; 32 ectodermal, easily recognized by 

 their transparency, and 12 other cells very much larger. The ectodermal cap is 

 notably extended and tends to envelope the endoderm. 



From this stage onward there is no question as to the identification of the dor- 

 sal and ventral faces or the anterior or posterior ends. As will presently appear, 

 my work, like that of Chabry (1887), entirely confirms the orientation adopted by 

 Van Beneden and Julin, though I cannot agree with them as to the fate of certain 

 individual cells. 



2. See tiger's System. 



Seeliger's (1885) later work was much less detailed and satisfactory with 

 regard to the orientation of the early cleavage stages, as Castle has shown. His 

 principal conclusions as to orientation are : 



(1) The first cleavage plane coincides with the median plane of the embryo, 

 hut neither anterior nor posterior, dorsal nor ventral can he recognized at this 

 stage (p. 48). 



(2) The second cleavage divides the egg into two smaller anterior cells and 

 two larger posterior ones (p. 48). 



(3) The third cleavage separates4 dorsal endodermal cells from 4 ventral ecto- 

 dermal ones; the two posterior ventral cells are larger than any of the others. 

 Structurally all these cells are alike (p. 49). 



