750 LIFE : OUTLINES OF GENERAL BIOLOGY 



The egg-ceil of the frog is about a tenth of an inch in diameter, 

 and is surrounded by a delicate cell-membrane, a tougher follicular 

 membrane, and, outside that, the jelly or albumen of the spawn. 

 There is a considerable quantity of yolk, which sinks to the lower 

 hemisphere of the egg ("the vegetative pole"), which has a lighter 

 colour than the upper hemisphere ("the animal pole"). When 

 fertilisation occurs, at the time when the eggs are liberated by the 

 female frog on whom the male is mounted, the vitelline membrane 

 rises from off the surface of the egg, which is thus able to rotate, 

 keeping the heavier vegetative pole always lowest. 



Where the spermatozoon enters is of importance in relation to 

 subsequent symmetry; for it marks the future ventral surface. 

 Diametrically opposite, there is a retreat of pigment into the egg, 

 forming a so-called "grey crescent", which marks the future dorsal 

 side. The animal pole will become the head-end and the vegetative 

 pole the tail-end. 



Segmentation is total and unequal. The upper hemisphere shows 

 numerous small cells; the lower hemisphere has fewer larger ceUs. 

 Between the two and internally there is the small blastula cavity 

 nearer the upper surface. 



The cells of the animal hemisphere divide more rapidly than those 

 of the vegetative hemisphere, and thus grow over the lighter- coloured 

 ceUs, which become less and less visible. At the centre of the grey 

 crescent there is a slight lip, below which an ingrowth of cells 

 gradually begins to estabhsh the inner germinal layer or endoderm 

 — ^which becomes the lining of the embryonic gut, the archenteron. 

 This intucking corresponds to the invagination of a blastula to form 

 a gastrula, and the lip corresponds to the dorsal side of the gastrula- 

 mouth or blastopore. This lip spreads to right and left of its first 

 appearance; the overgrowing dark ceUs cover the yolk cells more 

 and more towards the vegetative pole — till they are only visible as 

 a small circular patch — the "yolk-plug". The curved ends of the 

 spreading dorsal lip meet on the ventral side of the yolk-plug, so 

 that there is a complete dark ring round the yolk-plug. 



Meanwhile the ingrowth of endoderm-forming cells is going on, 

 especially on the dorsal side, forming a dorsal wall for the arch- 

 enteron, of which the yolk-laden cells are forming the lower portion. 

 This is really a gastrula formation, disguised by the relatively large 

 amount of yolk ; and during this process the whole egg rotates, so 

 that its origiaal vertical axis comes to be more or less horizontal. 



Along the dorsal-median line of the archenteron a supporting rod 

 of cells is now folded off; this is the beginning of the primitive 

 skeletal axis, the notochord; an endodermic rod, eventually replaced 

 by its mesodermic substitute, the vertebral column. 



Split off from each side of the archenteron is a layer of cells, the 

 mesoderm, which spreads between the ectoderm and the endoderm. 



