STUDIES IN MAMMALIAN EMBRYOLOGY. 521 



Bonnet, Fleischmann, a. o., who all agree in looking upon 

 this as the homologue of the blastopore, and who must conse- 

 quently regard the tissue which from here proliferates inwards 

 as in the first instance hypoblast.^ 



If for a moment we leave out of consideration the participa- 

 tion of the coalescing lips of the blastopore in the formation of 

 mesoblast, then the first question that presents itself to us is 

 this : How does the palingenetic hypoblast arising in the 

 region of the gastrula ridge reunite with the precociously se- 

 gregated portion of the hypoblast which is already spread out 

 below it as a closed sac, at all events as a continuous layer ? 

 If we suppose that instead of the solid gastrula ridge and 

 the shallow gastrula groove of mammals an ingrowth along the 

 lips of a wide-open circular or elongated blastopore could be 

 noticed, even then the direct fusion of this palingenetic hypo- 

 blast with that which had been cenogenetically developed as 

 a closed sac could not come about, and could not give rise to 

 an arrangement resembling the gastrula of Amphioxus or of the 

 Amphibia unless a circular patch of the cenogenetic hypoblastic 

 tissue were to disappear by which — on the supposition here 

 made — the palingenetic archenteron would be shut off from 

 the cenogenetic cavity of the umbilical sac. Only after dis- 

 appearance of the cell layers separating them the two cavities 

 combined would correspond to that of the archenteron of 

 Amphioxus. 



And yet not strictly because of the additional space which 

 the increase of food-yolk in the Hypotheria and Prototheria 

 (Huxley), as represented by the fluid contents of the umbilical 

 sac in the Eutheria, has called forth within the hypoblastic 

 dominions. 



We find an analogy to this circular patch of tissue separating 

 as a thin cell layer the two cavities just mentioned in the 

 membranes by which mouth and anus are primarily closed in 

 early developmental stages. These membranes undergo a 



' Possibly it is the presence of a continuous sheet of hypoblast below the 

 gastrula ridge which has hitherto been so much in the way of the recognition 

 of the real sequence iu the phenomena. 



