430 



mVERTEBEATA 



CHAP. 



TTUf' 



given the most recent and satisfactory account (1910). According 

 to him, during the ripening of the egg, one of the primitive germ 

 cells becomes embedded in its cytoplasm and degenerates, leaving, as 

 remains, a deeply staining oval body {nut, Fig. 339). When the 

 egg undergoes segmentation this body is found in one of the first 

 two blastomeres. In subsequent divisions it is not divided, but 

 always passes into one blastomere. When, however, the blastula 

 stage is passed and gastrulation has been accomplished, it is divided 

 between two cells, and these cells are the mother cells of the genital 

 organs. It thus appears that in Sagitta, as in the Crustacean Poly- 

 phemus (see p. 193), the cell destined to produce the genital organs 

 is distinguished, not by peculiarities of its nuclear .substance, but 

 by the possession of an extra store of nourishment. 



The next change which 

 occurs in the development is 

 the formation of the coelom. 

 This takes place by the de- 

 velopment of a pair of inwardly 

 directed folds in the archen- 

 teric wall, which start from its 

 anterior border, one on each 

 side of the middle line, and 

 grow backwards as a double 

 wall. In this way the archen- 

 teron becomes divided into a 

 median chamber, the digestive 

 tract or gut, and two lateral 

 chambers, one on each side, 

 the coelomic sacs. At first 

 all three chambers open into 

 a common undivided chamber 

 behind. 



At the same time the 

 shape of the embryo changes, 

 because as these folds develop 

 they use up a considerable 

 portion of the front part of the archenteric wall, which had projected 

 forward in a conical process. As a consequence, the anterior pointed 

 end of the embryo becomes changed into a broad, very slightly 

 convex surface, whilst the posterior end remains pointed. 



The mother cells of the genital organs are in contact with the arch- 

 enteric wall almost at the point where the folds originate, and as these 

 grow backward they carry the mother cells of the genital organs before 

 them, so that these primitive germ cells are carried back into the 

 hinder half of the embryo. There they pass into the coelomic division 

 of the archenteron on each side, round the bend of the fold, and each 

 then divides into an anterior and a posterior half {g^, g^, Fig. 340). 

 Whilst this has been occurring the whole embryo grows consider- 



FlG. 339. — Cross section of the blastula of 

 Sagitta biptmctata (?) in the 16-cell stage, 

 showing the determination of the mother cell 

 of the genital organs. (After Buchner. ) 



g, mother cell of the genital organs ; nut, remains 

 of nutritive cell embedded in the mother cell of the 

 genital organs. 



