Lilian V. Sam])snn 



479 



erated when the splanchnic layers of the two sides have met and form 

 the mesentery which suspends the now solid yolk-mass (Fig. B3, m). 

 After stage Y, the yolk-mass in this condition occupies the region of the 

 embryo between the more differentiated anterior and posterior portions of 

 the gut (Fig. F, stage YI, cf. Fig. C, stage YHI). 



Fig. B. Cross sections of three stages, to show the fate of tlie aroheiiteric cavity. 1, stage 

 II (drawn to smaller scale than 3 and S) ; 2, stage IV ; 3, stage VI. ac, ai-chenteric cavity 

 be, body cavity ; da, dorsal aorta ; m, mesentery ; ma, mesenterial artery ; pcv, posterior 

 cardinal vein; pvc, posterior vena cava ; Wd. Wolffian duct; y, yolk-mass. 



There are no intermediate stages between I and II to show the fate 

 of the blastopore and the history of the anus. In the posterior region of 

 an egg younger than stage I, before the notochord is formed, and in stages 



I and III, posterior to the notochord, there appears a dorsal evagination of 

 the archenteron, Avhich suggests the neurenteric canal, though not actually 

 traced into the neural tube in the stages in which it is hollow. In stage 



II the anus is formed, and in lY the differentiation 

 of the hind-gut is in progress. The lateral walls of 

 the posterior part of the broad low archenteric cavity 

 covered by the splanchnic mesoderm fold under until 

 they meet over the yolk-mass (Fig. Dl, 2, ac, g). 

 The process of infolding appears to take place from 

 behind forwards, forming a tube that opens posteriorly 

 by the anus, and anteriorly is continuous with the 

 archenteric cavity of the middle region. Thus the 

 cloaca, the portion of the gut from the anus to the 

 opening of the Wolffian ducts, is established, and 

 through all stages retains its lumen. 



x\s the cloaca is formed, the lumen of the archenteron is being obliter- 

 ated in the middle region, and in later stages, the cloaca leads to a rod 



Fig. C. Yolk-mass of 

 an embryo of stage 

 VIII, after the embryo 

 has been taken away, 

 seen from the dorsal 

 side, to show the rela- 

 tion between the yolk- 

 mass and the anterior 

 gut and posterior gut. 

 fg, foregut; hg, hind- 

 gut ; y, j'olk-mass. 



