34 ALEXANDER MEEK 



A. M. Marshall, 0. Schnlze, and 0. Hertwig have all drawn 

 attention to cases of fusion between the two cavities, cases 

 which indicate that sometimes the segmentation cavity may- 

 become converted into the forward part of the enteron. Such 

 have been regarded as abnormal, and upholders of the orthodox 

 view of the procedure which takes place within the egg of the 

 frog explain them as being due to faulty preservation or prepara- 

 tion. On the other hand, the accepted view is not supported 

 by a convincing series of figures. A stage is shown which 

 presents the segmentation cavity and the enteron. The next 

 stage is one which displays the enteron occupying the place of 

 the segmentation cavity and the latter as non-existent or 

 as a small cavity of the endoderm ventral to the enteron. 

 Even if such a change actually takes place it is plain that the 

 subsequent excavation and thinning of the ventral and lateral 

 walls of the endoderm restores the cavity to the enteron, and 

 the process could be explained as a variation of that produced 

 by an earlier fusion of the two cavities. With this in mind 

 it may be said, to begin with, that, if the segmentation cavity 

 sometimes produces the anterior part of the enteron and 

 usually disappears as a cavity enclosed by endoderm, the 

 nomenclature of the cavity is wrong and misleading. 



With a view to obtaining evidence of what actually occurs, 

 I requested the laboratory steward, D. C. Geddes, to prepare 

 serial sections of a large number of embryos. The eggs were 

 gathered in the neighbourhood of Newcastle. They were in 

 the process of segmentation, and those preserved six to seven 

 days after collection furnished all stages from the end of the 

 period of delamination to that in which the yolk-plug is 

 reduced to a small spot at the posterior end of the egg. These 

 I have now examined. 



I was quite prepared to find that the first-formed enteron, 

 the so-called segmentation cavity, was actually replaced by the 

 secondary or neurenteric enteron, but no such example pre- 

 sented itself. In all cases I found that the segmentation cavity 

 is converted l)y delamination into a primitive enteron distended 

 by dissolved yolk-products, and that it is put by fusion into 

 communication with the enteron formed underneath the 



