i GASTKULATION 47 



about mainly by a process of overgrowth similar to that met with in 

 other forms. 



A point of special interest is that the posterior portion of the arch- 

 enteric roof, in the neighbourhood of what will become later the mesial 

 plane, is without the distinct demarcation between ectoderm and 

 endoderm which is present elsewhere. This continuity of the two primary 

 cell layers apparently represents what is known in the Amniotaas the 

 primitive streak — a structure of great morphological interest which 

 will be discussed later on (G-oronowitsch, 1885 ; Jablonowski, 1898). 



While these processes are in progress the margin of the blasto- 

 derm elsewhere is also advancing over the surface of the yolk so as 

 gradually to enclose it. This enclosure of the yolk clearly corre- 

 sponds to what we have seen in other cases but it is difficult to be 

 quite certain as to how far it takes place by actual delamination and 

 how far this has been replaced by a secondary independent growth. 



It is only when the exposed surface of yolk becomes reduced to a 

 small round patch that the cell-margin bounding it shows inflection 

 all round so as to justify us in speaking of a blastopore. 



In the surviving Ganoid members of the group Actinopterygii 

 we find that the process of gastrulation, as is the case with other 

 characteristics, repeats conditions which are probably to be looked 

 on as ancestral. The gastrulation clearly belongs to the same 

 general type as that of Lampreys, Amphibians, and Lung-fishes. 

 That of Acipenser (Salensky, Bashford Dean, 1895) seems more 

 nearly to resemble that of Polypterus, and that of Amia (Bashford 

 Dean, 1896) and more especially Lepidosteus (Bashford Dean, 1895) 

 to point towards the mode of gastrulation found iu the modern 

 Teleosts. 



GASTRULATION IN AMNIOTA 



In comparing the process of gastrulation in the Amphibians and 

 Lung-fishes with that in Amphioxus or Polypterus we have seen that 

 there is a tendency for the greater part of the gastrular rim either 

 to become completely obsolete or to be, at least, greatly delayed in 

 its appearance, for example in the frog the greater part of the 

 gastrular rim makes its appearance only in a comparatively late 

 stage in the process of gastrulation. 



In the Amniota we find that this tendency has gone further. It 

 is only in the lowest group — the Beptilia — that an undoubted 

 gastrular lip is clearly recognizable. In the two remaining groups, 

 the Birds and Mammals, there is no convincing evidence that it has 

 not completely disappeared from development. 



Beptiles. — In a Beptilian egg before the commencement of 

 gastrulation the apical portion is covered by a blastoderm consisting 

 of a superficial layer of flattened ectoderm cells and, underneath 

 this, rounded lower layer cells which are separated by interstices 

 containing fluid. 



In the centre of the blastoderm (Fig. 29, A) an area, circular 



