EARLY EMBRYONIC DEVELOPMENTS. 



1007 



The yolk is formed of a peripheral layer of clear transparent cells— the 

 ectoderm, or epiblast— lining the vitelline membrane, and embracing a central 

 mulberry mass (morula) of polyhedral and granular cells— the endoderm, or hypo- 

 llast. The ectoderm shows at one of its points an aperture— the hlastopore, or 

 anus of Rusconi— into which penetrates and closes it, but without overlapping it, 

 a prolongation of the endoderm— the cork of Ecker. At this phase in its evolu- 

 tion, the yolk constitutes what is termed in comparative embryology, the meta- 

 gastnda. 



2. Passage of the Metagastrula to the Uterus, and Formation op 

 THE Vesicle and Blastodermic Layers. — As soon as the metagastrula has 

 reached the uterus, it commences to be transformed into a clear and transparent 

 vesicle, which grows rapidly, attaining in four or five days a diameter of from 

 8 to 9 mm., and constituting the blastodermic vesicle, or blastoderm. 



The appearance of the blastoderm is marked by the appearance of a fissure 

 that separates the ectoderm from the endoderm, leaving them only adherent at 

 a point corresponding to the blasto- 

 pore, which was already formed 

 towards the end of the third 

 day. 



Owing to the pressure of the 

 fluid that fills it and tends to ac- 

 cumulate, the ectoderm becomes 

 distended and fissures ; its cells 

 multiply, and it fiattens so as to 

 embrace a layer that, towards the 

 ninetieth hour, attains from '15 to 

 •17 mm. The endoderm, pressed 

 upon by the fiuid, is deformed and 

 spread over a point beneath the 

 ectoderm, where it becomes the 

 gastrodisc. The vesicle, formed at 

 this part of two superposed layers, 

 is still monodermic everywhere else 

 (Fig. 544) ; nevertheless, the cells 

 of the gastrodisc change in charac- 

 ter, becoming flattened, and in an 

 ovum of from 105 to 115 hours, 

 having a diameter of from -9 to 2 

 mm., they form an endothelial 



lining to the ectoderm to an extent which increases with the marginal growth of 

 the internal layer. 



But all the cells of the endoderm are not so transformed. In ova of five 

 days old, having from 2 to 4 mm. to the centre of the gastrodisc, some are 

 found which have preserved their primary characters of segmentation spheres, 

 and remain interposed between the ectoderm and endoderm— between the 

 external and internal layers. This residual cellular mass, which has escaped 

 the endodermic transformation, is the point of depr.rture of a third layer— the 

 mesoderm {mesoblast), or middle lager, which is already marked at this period by 

 a circular spot at the pole of the gastrodisc. It is this spot that has been known 

 since the time of Bischoff and Coste as the germinal area or streak. The germijial 

 —also named the embryonal— area is only at first the optical signification of the 



OVUM OF THE RABBIT, NINETY HOURS AFTER 



FECUNDATION. 



br, Cavity of the blastodermic vesicle; ep, epiblast, or 

 ectoderm, forming a complete sac ; hy, hypoblast, 

 or endoderm, forming the gastrodisc; zp, zonula 

 pellucida. 



