42 Growth in length of the Vertebrate Embryo 



the lower layers of the segmented germinal disc, or between them 

 and the largest yolk segments forming the basal part of the ovum. 



An entirely different account of gastrulation in the Pigeon's 

 egg has been given by Patterson, which if confirmed would 

 necessitate an entirely different interpretation of the facts so 

 far described. But it is extremely difficult to reconcile his 

 account of the early development of the bird's egg with the facts 

 of Mammalian and Reptilian embryology, and I am very doubtful 

 as to whether Patterson's conclusions can be accepted (v. my 

 paper on " Gastrulation in Birds," Q. J. M. S. Aug. 1912). 



The chief point in dispute concerns the exact mode of formation 

 of the lower layer or endoderm. That we must leave as uncertain, 

 only noting that the anterior edge of the germinal wall is reached 

 by the advancing edge of endoderm at about the time of the 

 laying of the egg. 



All investigators however are agreed upon the structure of 

 the blastoderm after laying and during the first hour of incuba- 

 tion, and we may now proceed to consider its later history. 



The freshly laid egg of Gallus if it has been fertilised, shows the 

 blastoderm lying on the upper pole of the yolk. In this the 

 following parts as already noted are discernible : 



Area pellucida (through which the nucleus of Pander can 

 often be seen) ; 

 Area opaca. 



Surrounding the margin of the area opaca a slight groove 

 usually runs separating the blastoderm from the surrounding yolk. 



Sections show that this blastoderm consists of a segmented 

 disc lying loosely on and connected with the sub-lying yolk, and 

 having an outer well-defined layer of cells, and under this a large 

 number of loosely arranged cells, which form an incomplete roof 

 to a distinct cavity between them and the yolk. This is the 

 subgerminal cavity. There is a good deal of variation in the 

 state of the egg when laid, sometimes it is further advanced than 

 this. Also some blastoderms may be broader, measuring quite 

 a millimetre more than others. In such cases there is a greater 

 attenuation of the cellular layers, and the subgerminal cavity is 

 much larger. 



The looser inner layer of cells when the section is taken along 



