18 



FORMATION" OF THE BLASTODERM. 



granular cells, which represents the remainder of the inner granular mass of the 

 blastodermic vesicle after the separation of the subjacent layer. The three 

 layers were believed by v. Beneden to represent the three permanent layers of the 

 blastoderm. But it has been conclusively shown (by Eauber and Kolliker in the 

 rabbit, and by Lieberkiihn and Heape in the mole) that the middle stratum of this 

 stage of development is not the permanent middle layer of the blastoderm, for it is 



Fig. 15. SECTION OF PART OF THE BLASTODERMIC VESICLE OF THE RABBIT AT six DAYS. (From 



E. van Beneden.) 



a, upper layer (Rauber's cells) forming with 6, the primitive ectoderm ; c, primitive entocleroi. 



cct. 



ent. 



Fig. 1C. A SECTION THROUGH PART OF A BILAMINAR BLASTODERM OF THE CAT. (E. A. S.) 



ect, primitive ectoderm ; ent, primitive entoderm ; z.p., thinned-out zona pellucida. 



before long converted into a layer of columnar cells which becomes closely applied to, 

 and soon blends with, Rauber's layer, so that the two now form but a single stratum, 

 which may be denominated the primitive ectoderm. 



Kolliker describes the cells of Rauber's layer as undergoing a kind of atrophy and gradual 

 disappearance, taking no part in the formation of the primitive ectoderm. The observa- 

 tions of Lieberkiihn and Heape, on the other hand, tend to support the view which has been 

 given in the text. 



Both layers, but especially the primitive ectoderm, are somewhat thickened near 

 the middle of the ovum over a circular or oval area, which appears slightly darker 

 than the rest of the ovum when this is viewed by transmitted light : it is known as the 

 embryonic area (fig. 1 7). The entoderm does not for a long time form a complete invest- 

 ment to the blastodermic vesicle, for as we have seen it commences to form near the 

 centre of the ovum, and only gradually grows round within the epithelial investment, 

 so that it terminates peripherally by a free border. In most mammals which have 

 been investigated, it has not completely enclosed the ovum when the mesoblast has 

 begun to form, but in the cat its growth appears to progress more rapidly, so that, 

 for a time, the blastodermic vesicle has two complete and distinct epithelial invest- 

 ments. Whether complete or incomplete, the two layers together constitute what is 

 known as the Ulaminar blastoderm (fig. 16), the formation of which marks a distinct 

 stage in the development of all the metazoa. 



But the blastoderm doss not long remain in the bilaminar condition. In the rabbit 

 and mole, and probably in most mammals, long before the primitive entoderm has 

 completely extended itself around the ovum, there occurs a considerable thickening 

 of the primitive ectoderm at one end the posterior of the somewhat oval embryonic 

 area. This thickening has at first a crescentic form, with the concave edge looking for- 

 wards, and from the middle of this edge a longitudinal thickening extends for a certain 

 distance towards the centre of the embryonic area. The thickening is produced by a 

 proliferation of the cells of the primitive ectoderm, and its consequent downgrowth 

 towards the primitive entoderm, and it is visible when the ovum is viewed from 

 above by transmitted light, as a streak or shadow which is known as the primitive 



