232 ZOOLOGY. 



(Anura) : it does not occur, for example, in the newts 

 (Urodela) whose ova have quite as much yolk. This is not, 

 therefore, a result of the yolk -laden condition of the ovum. 

 We have seen that the epiblast gives rise to epidermis and 

 nerve-tissue : in the frog a sort of precocious separation of 

 these two takes place, the outer layer of epiblast being the 

 epidermal layer, the inner the nervous layer. 



5. Differences in Early Development of Frog and 

 Amphioxus. Thus there are three kinds of differences 

 between the early stages of development in frog and 

 Amphioxus (1) a difference which is peculiar to the Anura 

 (precocious differentiation in the epiblast) ; (2) differences 

 which are mechanical results of the presence of yolk (e.g. the 

 absence of an archenteron cavity); (3) those which are 

 adaptations to the yolk-laden condition, e.g. the formation of 

 supplementary epiblast from yolk-cells. The distinction 

 between (2) and (3) may not be clear to the student at first. 

 We may explain it by saying that if we could, by a 

 delicate surgical operation, remove the yolk from a frog's 

 ovum, the mechanical effects of the yolk would disappear and 

 the ovum would, so far, tend to segment in Amphioxus 

 fashion; but the adaptational characters reside in the 

 nucleus and protoplasm and would not be changed by the 

 withdrawal of the yolk. 



6. Origin of Mesenteron. The mesenteron appears 

 as a cleft in the mass of yolk-cells, and starts from the 

 region of the blastopore, at the time when the blastopore is 

 only beginning to be marked out. As the epiblast comes to 

 close more and more completely round the blastopore the 

 split curves round it in the same way, so that it finally comes 

 to be a circular chink, and the yolk-cells that still show at 

 the surface are marked off by this chink as a cylindrical 

 plug (yolk-plug') in the centre of the blastopore (fig. 118, c). 

 The chink around this plug is the beginning of the 

 mesenteron. As it extends farther into the mass of yolk- 

 cells, the segmentation-cavity shifts round before it and is 

 eventually obliterated. The extension of the mesenteron 

 can be best understood by the study of fig. 118. 



