72 GENERAL SKETCH OF 



early ovum,, the protoplasmic covering of the yolk tended to 

 stream downwards and augment the disc, but now the blasto- 

 derm commences to grow up and over and to envelop the yolk 

 (epiboly, Fig. 22). With the commencement of this process the 

 blastoderm flattens, and the vertical height of the germinal 

 cavity is reduced to form a mere fissure, whilst the germ 

 becomes generally thinner, yet along one radius there is a 

 distinct cellular thickening, viz. the embryonic radius (Fig. 21), 

 and this is noticed many hours before the inflection of the inner 

 layer or hypoblast, which will be mentioned later. 



When the blastoderm covers about a quadrant of the globe 

 of the yolk, the rim or border is visibly thickened, forming, as 

 some authors name it, ' a true pad around the egg.' This 

 curious condition is usually explained thus : — the amount of 

 yolk prevents the formation of an ordinary ' embolic gastrula '- 

 stage in this case, hence the yolk has to be enveloped by 

 growth of the blastoderm over it. 



At this stage we may glance at the formation of the yolk 

 (deutoplasm) in such eggs. As a rule in pelagic eggs, the yolk 

 is a transparent homogeneous liquid through which a body like 

 an oil-globule, such as occurs for instance in the gurnard, freely 

 passes. In the earlier stages a different condition prevails : 

 thus in the roe or ovary the developing yolk is quite opaque, 

 composed of a number of spherules with intermediate proto- 

 plasm, as seen for example in the early ovum of the turbot 

 (Plate III., fig. 22). In such species as the cod, the yolk- 

 mass clears up before ripening, and the protoplasm collects 

 mostly on the surface, but that all the protoplasm is thus 

 removed is not yet proved. For instance, in some cases collec- 

 tions of protoplasm take place on the yolk after the closure of 

 the blastopore. In some eggs, as in that of the sole, a vesicular 

 condition of the surface of the yolk is caused by evident strands 

 of protoplasm cutting off portions of yolk, and in the herring- 

 tribe and murtenoids the whole yolk is thus divided. 



The transparent condition of the yolk is not found in the 

 demersal eggs and it is probably to be regarded as a special 

 protective adaptation to a pelagic habitat ; the pelagic fauna, 

 consisting of examples from most of the leading groups of 



