138 INVEBTEBKATA CHAP. 



undivided and help to complete the belt of broad flat cells, the other 

 parts of which are formed by the cells of the second quartette, to 

 which allusion has already been made. The other two cells each 

 divide into an anterior outer large cell, 3a 2al and 3a 2pl , respectively, 

 and a posterior inner smaller cell, 3a 2a2 and 3a 2p2 , respectively. The 

 last two eventually sink into the blastocoele and help, like the 

 similar cells of the second quartette, to form larval mesoderm, whilst 

 their two larger sisters enter into the formation of the stomodaeum. 



We are now able to take a more general survey of the cells which 

 enter into the formation of this structure. The front wall of the 

 stomodaeum is formed by the four cells 2b 2211 , 2b 2212 , 2b 222r and 2b 2221 . 

 Its right side is constituted by the cells 2c 221a and 2c 222a , and its left 

 side by the corresponding cells 2a 221a and 2a 222p . In its right anterior 

 corner we find the cells 3b 2al and 3b 2pl , in its left anterior corner the 

 corresponding cells 3a 2al and 3a 2pl . 



In the quadrants C and D the micromeres of the third quartette 

 divide, at first, similarly to those belonging to quadrant A, B. Thus, 

 taking 3d for example (and remembering that all said about it is equally 

 true of 3c), we find that it divides into 3d 1 and 3d 2 , and each divides 

 into anterior and posterior cells : of these 3d la and 3d lp become broad 

 and flat and remain undivided, and thus complete the band of this 

 kind of cell right round the egg ; though at a later period, as we shall 

 see, they form part of the posterior lip of the mouth. 3d 2a divides 

 into 3d 2al and 3d' 2a2 , and these cells will help to complete the hinder 

 wall of the stomodaeum in a manner to be described later. 3d 2p , 

 however, divides into an anterior and a posterior cell, 3d 2pa and 3d 2pp , 

 and both these cells undergo another similar division, so that we get 

 an antero-posterior directed line of four cells, 3d 2paa , 3d 2pap , 3d 2ppa , and 

 3d2ppp (Fig. 103, C). 



The last two of these cells constitute the rudiment of one of 

 the larval kidneys or archinephridia, the other being formed by the 

 corresponding cells in quadrant C. The more anterior cell of each 

 pair at a later stage sinks into the blastocoele and is transformed 

 into a flame cell or solenocyte, with a cavity, and a tuft of cilia 

 waving within it ; whereas 3d 2ppp forms the excretory tube and 

 remains in connection with the ectoderm (Fig. 106). 



Passing now to the fourth quartette we find that all four cells 

 divide radially, each giving rise to two daughters lying side by side ; 

 so that we have 4a a and 4a p , 4b r and 4b ! , 4c a and 4c p , and 4d r and 

 4d', as we pass round the egg. Of these all but 4d r and 4d' enter 

 into the formation of the gut wall : the last named will eventually 

 give rise to those longitudinal streaks of cells known as the 

 mesodermic or germinal bands. These bands will eventually 

 become hollowed out to form the coelom or true body- cavity, the 

 walls of which constitute the adult mesoderm. 



The fifth quartette divides also at first evenly, 5a into Sa 1 and 

 5a 2 , 5b into 5b x and 5b 2 , 5c into 5C 1 and 5c 2 , and 5d into 5d x and 5d 2 . 

 In quadrants A and B the division stops, but it goes on in quadrants 



