AVES. l6l 



which is continuous with a meshwork of granular protoplasm enveloping the 

 above described vesicles. The germinal wall is still continuous with the 

 hypoblast at its edge ; and close to the junction of the two the hypoblast at 

 first forms a layer of moderately columnar cells, one or two deep and 

 directly continuous with the germinal wall, and at a later period usually 

 consists of a mass of rounder cells lying above the somewhat abrupt inner 

 edge of the germinal wall. 



The germinal wall certainly gives rise to the hypoblast cells, which 

 mainly grow at its expense. They arise at the edge of the area pellucida, 

 and when first formed are markedly columnar, and enclose in their proto- 

 plasm one of the smaller vesicles of the germinal wall. 



In the later stages (fourth day and onwards) the whole germinal wall is 

 stated to break up into columnar hypoblast cells, each of them mainly 

 formed of one of the vesicles just spoken of. After the commencing 

 formation of the embryo the mesoblast becomes established at the inner 

 edge of the area opaca, between the germinal wall and the epiblast ; and 

 gives rise to the tissue which eventually forms the area vasculosa. It seems 

 probable that the mesoblast in this situation is mainly derived from cells 

 formed around the nuclei of the germinal wall, which are usually specially 

 aggregated close below the epiblast. Disse (No. 122) has especially 

 brought evidence in favour of this view, and my own observations also 

 support it. 



The mesoblastic somites begin to be formed in the lateral 

 plates of the mesoblast before the closure of the medullary 

 folds. The first somite arises close to the foremost extremity of 

 the primitive streak, but the next is stated to arise in front 

 of this, so that the first formed somite corresponds to the second 

 permanent vertebra 1 . The region of the embryo in front of the 

 second formed somite at first the largest part of the embryo 

 is the cephalic region. The somites following the second are 

 formed in the regular manner, from before backwards, out 

 of the unsegmented posterior part of the embryo, which rapidly 

 grows in length to supply the necessary material (fig. 103). As 

 the somites retain during the early stages of development an 

 approximately constant breadth, their number is a fair test 

 of the length of the trunk. With the growth of the embryo the 

 primitive streak is continually carried back, the lengthening of 

 the embryo always taking place between the front end of 

 the primitive streak and the last somite ; and during this 



1 Further investigations in confirmation of this widely accepted statement are very 

 desirable. 



B. III. I * 



