RENEWED STUDY OF GERMINAL LAYERS OF THE CHICK. 86l 



We have made measurements to test this point, and find that 

 the diminution of the space between the front end of the primi- 

 tive streak and the anterior border of the area pellucida is very 

 marked up to the period in which the medullary plate first be- 

 comes established. We can further point in support of our view 

 to the fact that the extent of the growth lateralwards of the 

 mesoblast from the sides of the primitive streak is always less in 

 front than behind; which would seem to indicate that the front 

 part of the streak is the part formed latest. Our view as to the 

 elongation of the primitive streak appears to be that adopted by 

 Gerlach. 



Our next stage includes roughly the period commencing 

 slightly before the first formation of a groove along the primi- 

 tive streak, known as the primitive groove, and terminating 

 immediately before the first trace of the notochord makes its 

 appearance. After the close of the last stage the primitive 

 streak gradually elongates, till it occupies fully two-thirds of 

 the diameter of the area pellucida. The latter structure also 

 soon changes its form from a circular to an oval, and finally 

 becomes pyriform with the narrow end behind, while the primi- 

 tive streak occupying two-thirds of its long axis becomes in most 

 instances marked by a light linear band along the centre, which 

 constitutes the primitive groove. 



In surface views the primitive streak often appears to stop 

 short of the hinder border of the area pellucida. 



During the period in which the external changes, which we 

 have thus briefly described, take place in the area pellucida, 

 great modifications are effected in the characters of the germinal 

 layers. The most important of these concern the region in front 

 of the primitive streak; but they will be better understood if we 

 commence our description with the changes in the primitive 

 streak itself. 



In the older embryos belonging to our last stage we pointed 

 out that the mesoblast of the primitive streak was commencing 

 to extend outwards from the median line in the form of two 

 lateral sheets. This growth of the mesoblast is continued 

 rapidly during the present stage, so that during the latter part 

 of it any section through the primitive streak has approximately 

 the characters of Ser. I, 5- 



