RENEWED STUDY OF GERMINAL LA\EKS OF TEE CHICK. 177 



Our observations commeuce at a stage a few hours after hatch- 

 ing, but before the appearance of the primitive streak. 



The area pellucida is at this stage nearly spherical. In it 

 there is a large oval opaque patch, which is continued to the 

 hinder border of the area. This opaque patch has received the 

 name of the embryonic shield — a somewhat inappropriate name, 

 since the structure in question has no very definite connection 

 with the formation of the embryo. 



Koller describes, at this stage, in addition to the so-called 

 embryonic shield, a sickle-shaped opaque appearance at the hinder 

 border of the area pellucida. 



We have not made any fresh investigations for the purpose of 

 testing Koller's statements on this subject. 



Embryologists are in the main agreed as to the structure of 

 the blastoderm at this stage. There is (PI. XIII, Ser. a, 1 and 

 ~) the epiblast above, forming a continuous layer, extending over 

 the whole of the area opaca and area pellucida. In the former 

 its cells are arranged as a single row, and are cubical or slightly 

 flattened. In the latter the cells are more columnar, and form, in 

 the centre especially, more or less clearly, a double row ; many of 

 them, however, extend through the whole thickness of the layer. 



We have obtained evidence at this stage which tends to show 

 that at its outer border the epiblast grows not merely by the 

 division of its own cells, but also by the addition of cells derived 

 from the yolk below. The epiblast has been observed to extend 

 itself over the yolk by a similar process in many invertebrate 

 forms. 



Below the epiblast there is placed, in the peripheral part of 

 the area opaca, simply white yolk ; while in a ring immediately 

 outside and concentric with the area pellucida, there is a closely- 

 packed layer of cells, known as the germinal loall. The 

 constituent cells of this wall are in part relatively small, of a 

 spherical shape, with a distinct nucleus, and a granular and not 

 very abundant protoplasm ; and in part large and spherical, filled 

 up with highly refracting yolk particles of variable size, which 

 usually render the nucleus (which is probably present) invisible 

 (a, 1 and 2). This mass of cells rests, on its outer side, on a 

 layer of white yolk. 



The sickle-shaped structure, visible in surface veins, is stated 

 by Koller to be due to a special thickening of the germinal wall. 

 We have not found this to be a very distinctly marked structure 

 in our sections. 



In the region of the area pellucida there is placed below the 

 epiblast a more or less irregular layer of cells. This layer is 

 continuous, peripherally, with the germinal wall ; and is com- 

 posed of cells, which are distinguished both by their flattened 



VOL. XXII. NEW SER. M 



