MARINE TELEOSTEAN DEVELOPMENT. 73 



animals, are as a general rule characterised by transparency of 

 the tissues. 



The yolk forms a store of nutriment which is little used in 

 the earlier stages of development, though there is no reason 

 to suppose that it is altogether inert, since the neighbourhood 

 of the layer of protoplasm separating it from the blastoderm 

 is a scene of considerable activity. In the later stages the 

 rapid shrinking of the yolk before the embryo leaves the egg- 

 shows how important it then is in affording nutriment for the 

 development of the embryo. 



All the changes are not readily seen without making 

 sections of the prepared eggs, especially as regards the origin 

 of the second primary germ-layer (hypoblast). Its exact 

 derivation has given rise to much speculation, but it would 

 appear mainly to arise as an infolding and ingrowth of the outer- 

 layer cells (epiblast), supplemented by periblast-cells. This 

 folding is seen at a very early stage, and after the disc has 

 flattened out it can be followed to the central region of the 

 animal pole, a region which corresponds with the embryonic 

 thickening formerly mentioned. The effect of this ingrowth of 

 cells is that the originally very definite outline of the embryonic 

 shield becomes irregular and finally disappears or passes im- 

 perceptibly away on all sides, except posteriorly. 



A typical section of a teleostean egg (e.g. that of cod) when 

 the yolk is about half-covered by the blastoderm shows a single 

 layered corneous outer stratum and beneath it a thicker mass of 

 cells, the former being the outer and the latter the inner layer 

 of the epiblast. Another layer of cells, the hypoblast, borders 

 the embryo towards the yolk. The third primary layer, or meso- 

 blast, has yet to be described. It arises in great part from the 

 lower-layer cells, that is, those seen very early under the 

 epiblast, and probably also in great measure from the hypoblast, 

 and the cells so derived soon become divided into longitu- 

 dinal sheets or masses. These three primary layers are clearly 

 distinguished in the early embryo, except at the extreme 

 hind region, in which they are all confluent in a mass of in- 

 different cells. At the extreme head-end, on the other hand, 

 there is still only epiblast and hypoblast, as the middle layer 

 does not extend into this region till later. 



