250 THE EMBRYOLOGY OF THE BLOW-FLY IN THE EGG. 



dence exists between the two. According to Huxley, the 

 archenteron in the crayfish is produced by an invagination of 

 the blastoderm close to the posterior extremity of the embryo, 

 on the dorsal surface of the yelk, and just in front of the 

 proctodaeal invagination, which Huxley terms the hind-gut. 



The relation of the mesenteron to the yelk is precisely similar 

 in the crayfish and fly embryos, and Huxley gives the following 

 account of the manner in which the mid-gut of the former is 

 developed : ' The embryo-crayfish remains only for a short 

 time in the gastrula stage. The blastopore soon closes up, and 

 the archenteron takes the form of a sac flattened out between 

 the epiblast and the food-yelk, with which its cells are in close 

 contact. Indeed, as development proceeds, the cells of the 

 hypoblast actually feed upon the substance of the food-yelk.' 

 He also adds in a footnote : ' Whether, as some observers 

 state, the hypoblast cells grow over and enclose the food-yelk 

 or not is a question which may be left open. I have not been 

 able to satisfy myself of this fact.' 



My own impression with regard to the development of the 

 mid-gut of the fly embryo is that the solid portion of the 

 invaginated hypoblast feeds upon, but does not enclose, the 

 food-yelk, but I am not able to state positively that such is the 

 case. The material which fills the mid-gut at a later stage of 

 development resembles the small granular cells which replace 

 the yelk, granular mesamcjeboids, but whether these have been 

 enclosed by the wall of the mid-gut, or have immigrated into 

 it through its epithelial wall, or whether they are developed 



Description of 1'late XIV. 



Fig. I. — A transverse section of an embryo five or six hours old, showing the relations 



of the primitive band, archenteron, amnion, and serosa. 

 Fig. 2, — A transverse section of an embryo ten hours old, showing the coelomic sacs 



and pericardial (segmentation) cavity. The amnion and serosa are not seen in 



the section from which the drawing was made ; they have been restored by dotted 



lines. 

 am, amnion ; am' , amnial cavity ; ar, archenteron ; c s, coelomic sac ; « c, neural 

 cord imbedded in parablast ; /, pericardial cavity ; pa, parablastic roof of the peri- 

 cardial cavity ; s, serosa ; y, yelk. 



