422 



INVEETEBKATA 



CHAP. 



The blastula stage may now be said to be attained. It consists 

 of the large cell I, of six dorsal rows of three cells each, capped by 

 three granular cells, of two ventral rows of four cells, and of an odd 

 cell VI in the mid-ventral line. 



Gastrulation now begins by the retiring of the cell I from the 

 surface. This cell may now be denominated E, as it is the mother 

 cell of the endoderm. This cell may, however, before its inward 

 movement begins, be divided into front and posterior cells. Whether 

 this has occurred or not it speedily does divide in this way, and then 

 by longitudinal planes into right and left halves, and then by further 

 transverse planes, till a mass of eight cells has been produced. 

 Then the three granular cells, if they have not divided before, now 



Fig. 333. — Two views of embryos of Ocdlidina ritsseola showing tlie process of gastrulation. 



(After Zelinlia.) 



A, dorsal view of an embryo in which the endoderm cell (I = E) is just sinlting inwards. In this 

 emhryo the granular cells ai, lt\, and IIi have each divided into right and left sister cells. Tliis is ex- 

 ceptional. This division usually does not occur until somewhat later. B, ventral view of a somewhat 

 later stage in the process of gastrulation. The granular cells have passed round the anterior end of 

 the emhryo on to the ventral surface. The endoderm cell has divided into anterior (Ej) and posterior 

 halves (E2), and is almost completely enclosed by ectoderm, gr, granular cells ; p.b, polar body. 



divide each into two cells lying side by side, and the two larger rows 

 of ventral cells are also subdivided by longitudinal planes giving rise 

 to four rows. The six granular cells become compressed into a mass 

 and are no longer arranged in a crescent form (Fig. 333). 



This mass is the rudiment of what we may term the primary 

 stomodaeum. It becomes forced from its anterior position on to 

 the ventral surface by the backward pressure of the cells on the 

 ventral surface — for these ventral cells continue their initial tendency 

 to push backwards and to pass on to the dorsal surface at the posterior 

 pole of the egg ; thus they force forward the rows of dorsal cells, and 

 it is this pressure which forces the anterior granular cells downwards 

 round the front end. To reach this stage requires twenty-four hours 

 from the time that the egg is laid. 



