444 



INVEETEBEATA 



CHAP. 



in the cell E, which is the mother cell of the endoderm. Before this 

 cell divides into Ej and Ej the nucleus shows a tendency to migrate 

 inwards and upwards, but when it divides the daughter nuclei lie at 

 the ordinary level. 



At the next division of the cells of the ventral family, mst divides 

 into an anterior cell st, which is one of the cells which form the 

 stomodaeum, and a posterior and lateral cell which is one of the 

 mother cells of the anterior mesoderm, prr similarly divides into 

 a stomodaeal cell o-t and a mesoderm mother cell fx. (Fig. 346). Ej and 

 Eg divide into right and left daughters which (following Plateau's 

 principle) assume a rhombic arrangement. The two tail cells c and y 

 each divide into an anterior and a posterior cell, and the cell D also 

 divides into right and left daughters. The nuclei of all four en- 

 doderm cells take up posi- 

 tions near the inner margin 

 of their respective cells, the 

 cytoplasm follows suit, and 

 so the process of gastrula- 

 tion is initiated (Fig. 348). 

 In the next stage P^ 

 divides into an anterior 



G-, and a 



G„ 



Fig. 348. — Optical median sagittal section through 

 the blastula of Ascaris megalocephala at the time 

 that the process of gastrulation is commencing. 

 (After Boveri.) 



The cells P4 and D are beginning to extend beneath E2. 



posterior v^j. 

 These two cells remain 

 quiescent and undergo no 

 further division until after 

 the larva is hatched and 

 the development of the 

 genital organs is begun. 

 Boveri, however, maintains 

 that sometimes, at any 

 rate, the hinder of these 

 cells divides into right and 

 left daughters which are 

 ectodermal, and that the two mother germ cells arise by a second 

 division of the front cell. The anterior pair of tail ceUs divide each 

 into two daughters lying side by side, so that a transverse row of 

 four is formed, but the posterior tail cells divide each into two, one 

 behind the other. The stomodaeal cells each divide into anterior and 

 posterior daughter cells, and slightly later the anterior mesoderm 

 cells follow suit. These mesoderm cells now pass into the blastocoele 

 and lie between ectoderm and endoderm on each side (Fig. 351). 



In the next cleavage both stomodaeal cells and mesoderm cells 

 are raised to four on each side. In this way two short mesodermal 

 bands are formed, each consisting of four cells, but each band 

 contracts so as to form a rhomb. The endoderm cells increase 

 to eight. The daughters of D (d and S) divide each into two cells, 

 lying side by side, so as to form a transverse line of four cells parallel 

 to the row formed by the front tail cells. 



