293 



evagination of the entoderm and 2) development from irregular ex- 

 tensions of entoderm into the mesoderm". The former method with 

 subsequent "deepening of the original evaginations accompanied by 

 constriction and consequent fusion" may give rise to tubules blind at 

 both ends. As to the meaning of the tubules he states himself "unable 

 to reach any satisfactory conclusion". 



It seems to be the concurrence of opinion that in apes and man 

 the yolk sac plays no role as an organ of nutrition. In view of this 

 fact it is important to call attention to the following results reported 

 in some of the other forms of placental mammals, particularly rodents 

 and insectivora; and in marsupials. Robinson (1892) says that in the 

 mouse and the rat "the yolk sac is the only organ of nutrition dur- 

 ing a certain period of development, and that it remains functionally 

 important during the greater part, if not the whole, of fetal life". 

 According to Hubrecht (1889) the yolk sac in the hedgehog plays an 

 important part as an organ of nutrition in the early stages of develo- 

 pment. OsBORN reports that in the yolk sac of the embryo-opossum 

 there is "little or no yolk to draw upon". 



The material at my disposal consisted of an embryo, for which 

 I am indebted to Dr. F. S. Mandelbaum, pathologist to Mt. Sinai 

 Hospital, New York City. This proved to be in an excellent state 

 of preservation. Study of serial transverse sections, stained with hemato- 

 xylin and eosin, reveals a perfectly normal development and excellent 

 differentiation of microscopic structure. The greatest length of the 

 embryo, corresponding to the neck-rump measurement, is 9,2 mm, and 

 according to Malls formula for determining the age of embryos, it is 

 approximately thirty (30) days old. The embryo shows three (3) gill 

 arches, and twenty-nine (29) somites (Fig. 1). The specimen was 

 fixed in 80 % alcohol. 



Macroscopically the umbilical vesicle is piriform, tapering to the 

 calibre of the pedicle toward the point of attachment. It showed 

 shght irregular superficial corrugations. It floated free between the 

 amnion and the chorion. It measured 5 mm in length, attaining a 

 maximum diameter of 3 mm ; the length of the pedicle was 6 mm. 



Microscopically the wall of the vesicle shows distinctly the three 

 zones described by Paladino : an external mesothelial layer, an internal 

 entodermal layer, and a middle connective tissue layer (Figs. 4, 5). 

 Neither in the external nor in the internal layer, however, is there 

 the slightest indication of hyaline cells ("sostanza ialina trasparente") 

 described and figured by Paladino for both these regions in the human 



