EMBRYOLOGY 389 



This process is completed about the time that the. outline of the 

 blastopore is completed, and thus we have produced a two-layered 

 embryo. The outside pigmented layer is the ectoderm and the 

 inside cells constitute the entoderm while between them is the cleft 

 representing the blastoccel which gradually becomes obliterated as 

 in Amphioxus. On the dorsal side the entoderm cells are in a layer 

 two or three cells deep, while on the ventral side they are much 

 larger, more irregular and many cells deep. A group of these 

 larger cells forming the yolk plug practically fills the blastopore. 

 These ventral cells are heavily laden with yolk and constitute a food 

 store upon which the embryo relies for some time to come. The 

 space within the entoderm cells is, of course, the archenteron, and it 

 may communicate with the exterior by a small cleft under the dorsal 

 lip of the blastopore. Thus we see that while the details of the 

 process differ considerably in Amphioxus and Rana, the result of 

 gastrulation in both cases, with certain modifications in the frog that 

 will be considered immediately, is to produce an embryo with an 

 ectoderm, an entoderm, an archenteron and a blastopore : a typical 

 diploblastic condition and the differences are in the main due to the 

 amount of yolk present in the frog's egg 



While we have not yet taken notice of the mesoderm, as 

 a matter of fact it has been formed for some considerable time, and 

 the above description of the gastrula is not strictly speaking accurate, 

 for when the stage just dealt with is reached a good deal of meso- 

 derm is already present. The simple diploblastic condition in Rana 

 is more theoretical or potential than actual, and the precocious 

 appearance of the mesoderm prevents its realisation in anything like 

 a complete form. 



If we examine closely a section through the embryo when gastru- 

 lation is commencing we shall find at the dorsal lip of the blastopore, 

 where the ectoderm and entoderm cells are in contact, another 

 small group of cells that are destined to form the mesoderm. They 

 are right in the bend of the lip between the ecto- and entoderm, 

 with both of which layers they are continuous. As the blastopore 

 lip extends laterally, these cells spread with it, so that when the 

 margins join up the primitive mesoderm cells also unite and thus 

 come to constitute a mesoderm ring within the lip of the blastopore. 

 The dorsal lip commences fairly high up, and as it extends, it first 

 moves down ventrally and daring this period it leaves behind it a 

 band of mesoderm, so that the ring is much deeper on the dorsal 

 side than on the ventral. Furthermore, the dorsal extension of the 

 entoderm carries the mesoderm with it so that it soon becomes 

 fairly extensive. At first more or less closely connected with the 

 entoderm, the mesoderm cells are gradually delimited fairly sharply 



