A HUMAN EMBRYO OF THE PRESOM1TE PERIOD. 



397 



of obliquely-cut endoderm extending from the 

 yolk-sac to unite with the allantoic stalk. 



Section 75: The floor-plate of the small ecto- 

 dermic vesicle lying between the body-stalk and 

 the chorionic membrane is narrower, now occupy- 

 ing only one-fifth of the perimeter; otherwise the 

 vesicle is about the same as in section 74. The 

 smaller adjacent vesicle has disappeared except 

 for a small area of its investing mesoclerm. In 

 the body-stalk of the main specimen the allan- 

 toic stalk is nearer the V-shaped evagination 

 of the endoderm of the yolk-sac (fig. 10, plate 2). 

 The endoderm appears to be a little thicker in 

 the area of evagination, which is perhaps due 

 to the oblique direction of the section. The 

 ventral portion of the yolk-sac was mechanically 

 injured, as was the case also in the two succeed- 

 ing sections. 



Section 76: The small ectodermic vesicle 

 which we have followed in the preceding sec- 

 tions differs here, in that it consists entirely of 

 thin, oblique cut ectoderm, owing to the fact 

 that the cavity is now contracted. The ecto- 

 derm is completely surrounded by mesoderm, 

 which shows a vacuolizat ion-process but no 

 blood-vessel formation. The smaller vesicle 

 has now entirely disappeared. In the main 

 specimen the endoderm has not quite unit oil 

 with the allantoic stalk. 



Section 77: The ectodermic vesicle is rapidly 

 contracting and shows an obliquely cut wall 

 surrounded by an irregularly vacuolated layer 

 of mesoderm. In the body-stalk of the main 

 specimen the allantoic stalk is directly con- 

 tinuous with the evaginated endoderm. 



Sections 78 and 79 : The ectodermic vesicle has 

 now disappeared and there are left only portions 

 of the investing mesoderm. In the main speci- 

 men the thickened V-shaped extension of the 

 endoderm of the yolk-sac represents in a clear 

 manner the way in which it evaginates to 

 become continuous with the allantoic stalk, as 

 shown in figure 9, plate 2. In the ventral pole 

 of the yolk-sac numerous foci of angiogenesis can 

 be recognized, the most advanced of which show 

 the presence of completed blood-vessels packed 

 with blood-cells. 



Section 80: This section was cut 20yu thick. 

 There is nothing left of the body-stalk except 

 its point of attachment to the yolk-sac. The 

 thickened area of the evaginated endoderm and 

 the small amount of mesoderm in the place of 

 the body-stalk can still be made out. 



Section 81 : Traces of the body-stalk can still 

 be recognized. The ventral part of the yolk- 



sac shows very good examples of early angio- 

 genetic foci. 



Section 82: All trace of the body-stalk has 

 disappeared. The dorsal pole of the yolk-sac, 

 however, can be readily distinguished from the 

 ventral pole by its distinct endodermal and 

 mesodermal layers, which are separated by a 

 cleft. Also the principal angiogenetic activity 

 is found in the ventral half. 



Sections 83 to 91: Ventral and dorsal poles of 

 the yolk-sac can be distinguished. Numerous 

 blood-vessels, partly filled with blood-cells, are 

 found. 



Sections 92 to 96: These sections are stained 

 with cresylechtviolett, which differentiates very 

 well the cells contained in the early blood-vessels 

 of the yolk-sac. In many of these vessel-forming 

 masses the endothelium-lined lumen and its con- 

 tained cells are very well differentiated. 



Sections 97 to 101 : Heavily stained with hema- 

 toxylin, eosin, aurantia, and orange g. A con- 

 siderable amount of granular coagulum is pres- 

 ent in the yolk-sac and resembles very closely 

 the coaglum existing in the exocoalom. 



Sections 102 to 105: Stained in hematoxylin 

 and eosin. The yolk-sac is becoming smaller and 

 the sections through its wall are therefore some- 

 what oblique. This facilitates the study of the 

 young blood-vessels, some of which exist in the 

 form of a small plexus. In addition to the 

 granular coagulum there is found in the lumen 

 of the yolk-sac a few cells resembling small, 

 mononuclear leucocytes. It is possible that 

 these are displaced cells, as these sections are 

 somewhat broken. 



Sections 106 to 110: Deeply stained with hema- 

 toxylin, eosin, aurantia, and orange g. It is 

 possible that these sections are out of their order 

 and should perhaps have been placed before the 

 preceding four sections. They show consider- 

 able angiogenetic activity. 



Sections 111 to 115: Stained by the Biondi- 

 Ehrlich method. These sections show very 

 clearly the process of differentiation of the meso- 

 derm of the yolk-sac into endothelium and con- 

 tained blood-cells. 



Sections 116 to 124: The first 4 of these are 

 stained in safranin and light green; the last 4 

 in hematoxylin, eosin, aurantia, and orange g. 

 In these sections the yolk-sac rapidly rounds off 

 and disappears. The coat of mesoderm is thicker 

 in this region and shows well-developed vessels. 

 This point corresponds to the ventral pole of the 

 yolk-sac. 



Sections 125 to 277 contain only the chorion. 



