292 



(1891) "the yolk sac is merely a rudimentary structure without func- 

 tion .... morphologically significant but functionally nil" ; that in apes a 

 characteristic of their development is "the early separation of the yolk 

 sac from the chorion. The former takes no part in the nutrition of 

 the embryo and must therefore be regarded as a rudimentary organ". 



Paladino made a comparative study of the yolk sacs of higher 

 vertebrate forms including the dog, cat, guinea pig, chick, and a single 

 human specimen. He gives the age of this human embryo as about 

 a month ; according to this own statements, and judging from his 

 descriptions, which indicate degeneration, this specimen was in a poor 

 state of preservation. He describes a similarity of structure for the 

 wall of all sacs studied, in that it consisted of three distinct layers: 

 a layer of internal epithelium ("limite interno"), an intermediate, very 

 vascular connective tissue layer ("zona media"), and an external endo- 

 thelial layer ("limite esterno"). He describes the first layer as having 

 "more the aspect of a secretiug than an absorbing membrane". He 

 denies for the human yolk sac any nutritive function. According to 

 him, "The umbilical vesicle^ represents an organ which besides being 

 the expression of the tenacity of hereditary form, difi'erentiates into 

 a gland in which possibly the production of the secreting parts is 

 accentuated so as to recall some of the internal secretions of the he- 

 patic glands". 



Meyer gives an interesting review of the early literature on the 

 human yolk sac and calls attention to the fact that "most of the early 

 investigators ascribed nutritive and haematogenous functions to it". 

 The basis for Meyer's investigation is a series of eighteen normal 

 umbilical vesicles from the collection of Dr. Mall. The vesicles are 

 from embryos ranging in age from 11 to 110 days. The series in- 

 cludes three embryos of approximately thirty (30) days' age, in all of 

 which "tubules" are found. As the result of a very complete study, 

 Meyer reports "that these tubules make their appearance during the 

 second week, reach their greatest development by the fourth or fifth 

 week and then gradually disappear by the eighth or ninth week". 

 This is slightly at variance with the results of Graf Spee according 

 to whom the tubules go on increasing and spreading over the yolk 

 sac until between the eighth and ninth weeks. But this discrepancy 

 may have little significance, as Meyer suggests, for he has found "the 

 widest variations as to the presence, structure and size of these tu- 

 bules". 



Meyer distinguishes two modes of formation of the tubules: "1) 



