34i F. M. BALFOUE. 



able or exact, I think that we both find and should expect 

 to find that it is, roughly speaking, fairly so. 



Thus, the muscles, internal skeleton, and connective tissue 

 are always placed in the adult between the skin (epidermis) 

 and the epithelium of the alimentary canal. 



We should therefore expect to find them, and, as a matter 

 of fact, Ave always do find them, developed from a middle 

 layer when this is present. 



The upper layer must always and does always form the 

 epidermis, and similarly the lower layer or hypoblast must 

 form a part of the epithelium of the alimentary canal. 

 A full discussion of this question would, however, lead me 

 too far away from my present subject. 



The only other point of interest which I can touch on in 

 this stage is the commencing closure of the alimentary canal 

 in the region of the head. This is shown in PI. XIII, fig. 

 6 a, 6 b, and PI. XIV, lb, n. a. From these figures it can be 

 seen that the closing does not take place as much by an in- 

 folding as by an ingrowth from the side walls of the alimentary 

 canal towards the middle line. In this abnormal mode of 

 closing of the alimentary canal we have again, I believe, an 

 intermediate stage between the mode of formation of the 

 alimentary canal in the Frog and the typical folding in which 

 occurs in Birds. There is, however, another point in refer- 

 ence to it which is still more interesting. The cells to form 

 the ingrowth from the bottom (ventral) wall of the alimentary 

 canal are derived by a continuous fresh formation from the 

 yolk, being formed around the nuclei spoken of above (vide 

 p. 329). All my sections show this with more or less clear- 

 ness, especially those a little later than fig. 6 b, in which the 

 lower wall of the alimentary canal is nearly completed. This 

 is the more interesting since, from the mode of formation of 

 the alimentary canal in the Batrachians, &c., we might expect 

 that the cells from the yolk would take a share in its forma- 

 tion in the Dog-fish. I have not as yet made out for certain 

 the share which is taken by these freshly formed cells of the 

 yolk in the formation of any other organ. 



By the completion of its lower wall in the way described, 

 the throat early becomes a closed tube, its closing taking 

 place before any other important changes are visible in the 

 embryo from surface views. 



A considerable increase in length is attained before other 

 changes than an increase in depth of the medullary groove 

 and a more complete folding off" of the embryo from the 

 blastoderm take place. The first important change is the 

 formation of the protovertebicc. 



