VII ANNELIDA 139 



C and D ; in these 5ci and Sc^ divide into 5c" and Sc^^, and into 5c^^ 

 and 5c22, respectively ; and the same thing happens to 5d' and Sd^. 



This greater growth in the hinder quadrants of the egg, which 

 occurs both in the 3rd and 5 th quartettes, has the effect of pressing 

 the pairs of cells ba} and 5a2, and 5h^ and Sb^, out of their original 

 arrangement of two lines converging to the lower vegetative pole of the 

 egg, into a position of two lines parallel to one another, and they will 

 eventually form the sides of the mid-gut (Fig. 106). 



The residual macromeres 5A, 5B, 5C, and 5D, lastly divide, each 

 into two equal daughters ; 5C and 5A into anterior and posterior cells, 

 5B into right and left cells by radial divisions ; but 51) divides into 

 an inner and an outer cell 5I)i and 50^, and the outer 5D^ divides 

 again into SD^^ and oD^^, so that here, as in the fifth quartette, we 

 have increased multiplication of cells in the hinder part of the egg. 



These divisions complete all the divisions of cells which take 

 place in the flattened plate-like blastula. We have, as we have 

 already seen, 40 cells of the first quartette. Of the second we have 

 8 stomodeal, 4 larval mesoderm, and 26 cells forming the belt of 

 flattened cells ; i.e. 38 in all. Of the third quartette 8 enter into 

 the formation of the stomodaeum, and 4 form larval mesoderm, 4 form 

 larval kidneys, and 4 form ventral ectoderm (3c^p'''' and ScP'^p, + Sd^P''^ 

 and 3d^p^p, respectively), making 28 in all. The fourth quartette 

 contains only 8 cells, the fifth 12, and there are 9 residual 

 macromeres. So that the grand total of all the cells at this stage is, 

 40 + 38 -H 28 + 8 + 12 + 9 ; i.e. 135 in all. 



At this point of development invagination of the cells of the 

 lower surface begins, and the blastula is converted into a gastrula 

 which, in virtue of its apical plate and its four groups of prototrochal 

 cells, may be already termed a Trochophore. 



We shall now study how invagination is brought about. The 

 nine residual macromeres form a plate at the vegetative pole. The 

 two cells forming the centre of this plate, namely SD^ and 51)^^, rise 

 upwards into the blastocoele, in consequence no doubt of altered 

 chemical conditions here, that is, of altered cytotaxis. As the 

 centre of the plate thus sinks in, two lateral ridges of cells become 

 prominent and outline the edges of the indentation so formed ; in a 

 word they outhne the blastopore. These lateral ridges are, on the 

 right side, 5b\ 5b2, 4c% 5cl^ and Sc" ; on the left side 5ai, 5a.\ 4a^ 

 5di2 and 5d". 



The blastopore takes on the form of an oval opening, elongated 

 in an antero-posterior direction. The front of the blastopore is formed 

 by the cells 4b'' and 4b', and the hinder end at this stage by the cells 

 4d'' and 4d'. The centre cell in each row (4a=' and 4c^) approaches its 

 opposite partner and so the ridge of the oval is converted into a 

 figure of eight. These two latter cells finally meet one another and 

 the oval opening is thus cut into two openings, the primitive mouth 

 and the primitive anus respectively. 



The primitive mouth persists, but the primitive anus is temporarily 



