ORIGIN OF THE ARCHENTERON. 



249 



veloped, which ultimately closes above and forms the metenteron. 

 The metenteron then sinks into the yelk, and its blind posterior 

 extremity, as I believe, eventually unites with the ventral 

 proctodaeal involution ; but on this point I have no actual 

 evidence, as I have been unable to trace the union of the 

 hind gut and proctodaeum, although it is clear that if I am 

 right in the interpretation of the observations of myself and 

 others, such a union must occur. 



The mid-gut, that is, the part of the intestine in front of the 

 blastopore, first turns backwards beneath the metenteron 

 (Fig. 3), but its hinder blind extremity afterwards turns forwards 



Fk;. 39. — An ideal section of an embryo constructed from transverse sections, showing 

 the relations of the archenteron, stomod.-eum, proctodeum, and cct'loniic sacs. 

 The parablast of the ca'loniic sacs and pericardial (segmentation) cavity, is repre- 

 sented l)y a chain of cells : a, amnial sac ; c c, cuelomic sac of the right side ; i a, 

 space between the amnion and serosa ; p, pericardial cavity ; pr, proctoditum ; 

 st, stomodix;uni. 



and forms a solid cellular growth, which penetrates the yelk 

 and ultimately becomes the mid-gut (Fig. 39). 



The Malpighian tubes are developed as sacculi of the arch- 

 enteron, one on either side of the blastopore, but subsequently 

 travel back as far as the posterior extremity of the archenteric 

 invagination, so that they open into it immediately behind its 

 solid cellular portion. 



If we compare the development of the archenteron of the fly 

 embryo, as I have described it above, with that of the crayfish, 

 as described by Huxley,* it will be seen that a close correspon- 



* Huxley, T. H., 'The Crayfish : an Introduction to the Study of Zoology, 

 Internat. Sc. Ser., vol. xxviii., London, Paris and Berlin, 1880. 



17 — 2 



