62 



which "können sehr wahrscheinlich zu jeder Zeit durch Abschnürung 

 wieder einkernige Zellen bilden, welche die Bedeutung von indiöerenten 

 Wanderzellen besitzen" (p. 521). I have found no evidence of such 

 origin of normoblasts in my material. 



Seeing that the umbilical vesicle still functions at this comparatively 

 late stage of development (from the standpoint of hematopoiesis) as a 

 hematopoietic organ, and that hematopoiesis is almost nil or at least 

 very meagre within the embryo, the sac seems to have a very important 

 function in contributing the earlier types of the fetal blood cells. These 

 are washed into the liver, heart and vessels of the embryo where as 

 megaloblasts and normoblasts they extensively proliferate. 



Significance of human Umbilical Vesicle from the 

 Standpoint of Phylogeny. 

 This brings us to a brief discussion of the bearing of the facts 

 above outlined for the umbilical vesicle on the question of the phylogeny 

 of the higher vertebrates. On the basis of very numerous facts gathered 

 from comparative embryology, Hubrecht (21) argues cogently against 

 the generally accepted opinion regarding the ancestry of the vertebrates, 

 viz. that they are descended from ancestors with meroblastic yolk- 

 laden eggs, i. e. sauropsida and monotremes. Hubrecht attempts to 

 show on grounds of early ontogenetic phenomena in mammals, that the 

 yolk sac as seen in the anthropomorphae and man — where the blastocyst 

 is surrounded by a decidua capsularis, thus excluding osmotic absorption 

 and transportation of materials passed from the uterus to the extra- 

 embryonic coelom — is the primitive type, and has originally only a 

 hematopoietic significance. The same has been suggested by Saxer ('96) 

 and Spee ('96). Hubrecht looks upon the human and simian umbilical 

 vesicles as essentially blood-forming organs, and suggests that the "hemato- 

 poietic significance of the surface of that part of the endoderm which 

 has grown out into an embryonic appendage, and has been styled the 

 umbilical vesicle, may be its chief raison d'etre" (p. 85). He interprets 

 the yolk sac as a "hernia-like expansion of part of the gut" that in 

 primates had first a hematopoietic significance, secondarily acquired 

 significance in omphaloidean placentatiou in many mammals (insectivores 

 and Carnivora), and finally cooperates (in sauropsida) toward the trans- 

 port of a reserve yolk substance accumulated against the inner surface 

 of the network of blood vessels. According to Hubrecht the question 

 arises "if this significance of the vascular area on the umbilical vesicle 

 of the higher vertebrates is not older than that other property which 

 it has assumed in the meroblastic eggs of sauropsida, viz., the pro- 



