42 EMBRYOLOGY OF THE LOWER VERTEBRATES oh. 



Gtmnophiona. — There are certain Amphibians in which the 

 telolecithal condition of the egg is so pronounced as to lead to a 

 condition nearly approaching the meroblastic. Any such forms 

 occurring either amongst the Dipnoi or the Amphibia must necessarily 

 be of great importance owing to the fact that these groups are less 

 far removed than are any other Vertebrates, from the line of descent 

 of the Amniota and that, in consequence, the study of their develop- 

 ment may be expected to throw light upon features occurring in the 

 meroblastic eggs of Amniotes. 



Amongst Amphibians of this type the Gymnophiona alone have 



been subjected to careful study (P. and E. Sarasin, 



/ 1887-1893 ; Brauer, 1897). The following sum- 



■•.. ^_^ ..•'"' niary of the main features in their gastrulation 



processes is based on Brauer's description. 



The egg of Hypogeophis shows at the period 

 '"•■.. /• — \ ..••'' preceding gastrulation a round patch of micro- 

 meres, or blastoderm, covering \-\ of the 

 surface of the egg in the neighbourhood of its 

 apical pole. Gastrulation commences with the 

 posterior edge of the blastoderm losing its forward 

 curvature and becoming curved backwards (Fig. 

 ■■{Z).-"' 26), the curved part of the edge becoming 



sharply demarcated by the formation of a slight 

 cleft-like invagination of the egg-surface — which 

 is deepest in its centre and shallower towards 

 its extremities. In front of this invagination the 

 Fiu. 26. — Successive superficial (ectodermal) cells of the blastoderm 

 stage^ofgastaiiarHp take on a distinctly columnar form. The edges 

 seen in plan. (After of the blastoderm apart from the line of invagina- 

 Brauer, 1897.) tion are in the meanwhile gradually spreading 



outwards over the yolk. As shown in Fig. 27, 

 A, the cells (g.l) forming the anterior wall of the invagination are 

 columnar in form, and the fine-grained character of their yolk makes 

 their general appearance resemble that of the ectoderm cells. This is, 

 however, to be taken, not as meaning that they really are of ectodermal 

 nature but rather merely as an indication of active metabolism 

 associated with active growth. The invagination-groove gradually, 

 by backgrowth and ingrowth of its lateral portions, assumes a more 

 pronounced backward curvature (Fig. 26) taking first the shape of a 

 crescent, later of a horseshoe and finally of a closed ring. 



The central part of the groove almost from the beginning 

 increases rapidly in depth so as to form a narrow cavity — the 

 archeuteron — which extends forwards. The roof of this cavity is 

 formed of cells agreeing in their fine-grained protoplasm with those 

 of the ectoderm, while its floor on the other hand is composed of cells 

 which in their coarse-grained character resemble rather the yolk -cells. 

 In front of the archenteron are the irregular remains of the 

 segmentation cavity and a communication becomes established 







A 



