368 EMBRYOLOGY 



whicli soon narrows. At an early period two large cells, 

 the genital cells, become noticeable at the bottom of the 

 archenteric invagination, directly opposite to the blastopore. 

 A plane passing between these two cells would correspond 

 to the future plane of symmetry. In the course of further 

 development the genital cells withdraw from epithelial con- 

 tinuity in the wall of the archenteron, passing into the 

 archenteric cavity. Here they divide so that four 

 genital cells lying in the transverse axis can be distinguished 

 (Fig. 161 B). Of these the two middle ones represent the 

 fundaments of the two testes, the two outer, on the other 

 hand, those of the ovaries of the two sides. In the anterior 

 widened portion of the archenteron the formation of two 

 folds now takes place from in front backwards ; these push 

 the genital fundaments before them (Fig. 161 B), and by 

 their development the archenteron is divided into three 

 spaces lying side by side, the middle one of which represents 

 the cavity of the mid-gut, the two lateral ones, on the 

 other hand, those of the paired coelomic sacs. ^ 



While the embryo now increases in length, the blastopore 

 closes and the permanent mouth-opening breaks through, 

 the latter being accompanied by the development of the 

 fore-gut, which probably arises as an ectodermal invagina- 

 tion (Fig. 161 C, st). The middle one of these three 

 previously formed diverticula acquires in this way an open- 

 ing anteriorly. In the view from the dorsal side (Fig. 162 

 A) the blastopore and permanent mouth appear to lie 

 directly opposite each other, but side views show that the 

 blastopore is moved a little, as it seems, towards the ventral 

 side of the embryo, so that accordingly the permanent 

 longitudinal axis occupies a position oblique to the chief 

 axis of the gastrula. 



With their further growth in length the two folds are 

 pushed farther and farther backwards (Fig. 161 C). In this 



1 [According to Jourdain, the two evaginations of the archenteron do not 

 produce the ca'lom, but their cavities disappear, and at the same time 

 between ectoderm and entoderm there is formed in the mesoderm a 

 fissure, which becomes the permanent body cavity. This statement 

 contradicts those of the authors mentioned above.— K. ] 



