36 



PRIMITIVE STREAK 



layer cells of the head-plate and passing behind on to the wall of the yolk-sac. The 

 ring is closed behind when the hind-gut becomes cut off from the yolk-sac. It is 

 apparently directly related to the development of the blood-vessels. A similar ring 



FIG. 52. HYPOTHETICAL STAGE OF THE HUMAN OVUM IMBEDDED IN THE DECIDUA, SOMEWHAT 

 YOUNGER THAN PETERS' OVUM, THE TROPHOBLAST IS GREATLY THICKENED, AND LINED WITH 

 MESODERM, WHICH SURROUNDS ALSO THE EMBRYONIC RUDIMENT, WITH ITS YOLK-SAC AND AMNIO- 

 EMBRYONIC CAVITY. (T. H. Bryce,) 



The embryonic rudiment is proportionally on too large a scale. 



was described by the same observer in the shrew, and by Bonnet in the sheep, 

 but it has not been found in other mammals. 



The primitive streak (although this feature is not marked in any of Hubrecht's figures of 

 Tarsius) becomes deeply indented by a furrow called the primitive groove, and it is to be observed 



embryonic ectoderm 

 \ 



ectode 



mesoderm 



r<fer? x -r-- . _ chorion 



yolk-sac 



amnion 

 connecting stalk 



allanlois 



FIG. 53. MEDIAN LONGITUDINAL SECTION OF AN EARLY HUMAN OVUM 0'4 MM. IN LENGTH. 

 (After Graf v. Spee, from Kollmann's Entwickelungsgeschichte). x 27 diameters. 



that in Graf v. Spec's embryo of 2 mm. (fig. 59), the mesoderm-sheets are represented as 

 partially subdivided into two lamellse, one connected with the ectoderm, the other with the 

 entoderm. This feature, also seen in a number of lower forms, has been interpreted by Hertwig 

 as a vestige of an original development of the mesoderm by ccelomic pouches (see p, 46). 



