Formation of the Segmentation Cavity 13 



The segmentation cavity which is so large and conspicuous an 

 object in E becomes smaller and appears to be obliterated by 

 gradual compression owing to the advance of cells caused by the 

 expansion of the gastrulation slit. 



At the same time it must be noted that there is some uncertainty 

 as to whether the whole of the segmentation cavity is obliterated 

 in this way, or whether some of it may not remain and become 

 merged with the new archenteric cavity derived from the slit, and 

 thus in fact help to form the future gut cavity. 



There is no doubt that the wall between the original segmenta- 

 tion cavity and the slit cavity becomes very thin. According to 

 Hertwig this wall breaks down in the frog. According to Brachet 

 it breaks down sometimes in Rana temporaria though not always, 

 but in Siredon it always does break down, as according to Brauer 

 it does in Hypogeophis. Hence we may say that in Amphibia 

 the segmentation cavity either disappears by compression of its 

 walls or it may become confluent with the anterior region of the 

 archenteron. 



To compare this condition with Amphioxus we may refer to 

 the diagrams of sections drawn in corresponding positions. It 

 will be seen that a certain amount of invagination occurs. A 

 mark placed in the yolk plug at X should appear within the gut 

 cavity at a later period, Fig. 8. 



There are no essential differences with respect to gastrulation 

 in the many genera of Anura and Urodela which have now been 

 studied. 



In the case of the Gymnophiona however we have an egg 

 which is distinctly nearer to the meroblastic type than is that of 

 either of the other two Amphibian orders. It is almost a centro- 

 lecithal egg. 



In the egg of Hypogeophis, which has been very well studied 

 and described by Brauer, the segmentation is partial; but there 

 is a very rapid superficial differentiation so that a circular blasto- 

 pore is formed very much as in the other Amphibia. The 

 segmentation at one time shows a cap of small cells upon a mass 

 of still unsegmented yolk, with here and there a nucleus near to 

 the segmented pole. There is no very well marked segmentation 

 cavity, but small cavities appear here and there among the small 



