540 



SCIENCE 



[N. S. Vol. XLII. No. 1085 



In many embryos the circulation after having 

 begun may stop for a time and then later be 

 reestablished, the vessels having persisted in a 

 normal condition. Thoma's so-called laws of 

 vessel formation are, therefore, rudely violated 

 by the development of the vascular system in 

 these embryos. 



The vessels arising from independent mesen- 

 chymal cells in the space of the blastocele in 

 the teleost yolk-sac entirely overthrow any no- 

 tion that vessels arise ontogenetically as por- 

 tions of the celomic epithelium. The vascular 

 lumen is originally continuous with the pri- 

 mary body cavity, the segmentation cavity, and 

 never with the secondary body cavity, or 

 celomic cavity. 



The fourth class of cells wander out from the 

 embryonic body somewhat later than the three 

 former types. These are small circular cells 

 with short pseudopod-like processes. They 

 move very slowly, but finally collect into 

 groups on the posterior and ventral regions of 

 the yolk-sphere. 



The round cells wander away only from the 

 caudal region of the embryo and probably are 

 derived from the so-called intermediate cell 

 mass which is the anlage of the red blood 

 corpuscles in the fish embryo. 



The groups of round cells are slow in their 

 differentiation but just before the circulation 

 of the blood begins, they are seen to be cir- 

 cular erythroblasts. The observer may follow 

 the disappearance of the islands of cells one 

 by one as they are enclosed by the vessels and 

 swept into the circulating stream. About the 

 fifth day these circular erythroblasts become 

 flattened ellipsoidal erythrocytes filled with 

 hemoglobin, the typical red blood corpuscle. 

 The complete change from wandering, more or 

 less globular mesenchymal cells into typical 

 hemoglobin-bearing corpuscles may be followed 

 in the living yolk-sac. 



In several instances the body proper of the 

 embryo failed to develop or else degenerated 

 very early, yet the yolk-sac formed or persisted 

 with numerous blood islands fully differen- 

 tiated. 



The embryos in which there has been no cir- 

 culation of the blood form the blood islands 



from the wandering cells on the yolk-sac, and 

 the constituent elements of these islands differ- 

 entiate perfectly and may maintain their red 

 color for many days. Yet they never leave the 

 locality in which they have differentiated. The 

 fully formed red blood corpuscles have little 

 if any power of migrating. When the ob- 

 server can be positive that the blood has never 

 circulated, and this requires very consistent 

 watching, the blood islands of the yolk-sac are 

 always limited to certain regions, and never 

 occur so far anteriorly on the ventral surface 

 of the yolk as to reach the venous end of the 

 heart. 



Finally, we may consider the study of the 

 developmental products of the early wandering 

 mesenchymal cells on the yolk-sac of the 

 Fundulus embryo as a problem of cell lineage 

 followed to its ultimate end. The primordial 

 mesoderm cell or cells carry within their bodies 

 all the potentialities of the mesoderm and may 

 give rise to a series of cells which are capable 

 of developing muscle, cartilage, bone, connec- 

 tive tissue proper, blood cells, vessels, etc. 

 Yet after a few cell generations the individuals 

 in the series derived from these early cells con- 

 taining all the mesodermal potentialities no 

 doubt become somewhat limited as to their 

 potentialities. In a certain generation there 

 may be definite cells more or less generally 

 distributed which possess the capacity to give 

 rise to muscle cells, but to no other type of 

 mesodermal tissues. Still later in develop- 

 ment these cells may become even more limited 

 in their developmental capacities and thus 

 have the power to produce only a certain type 

 of muscle cell and no other type. 



Collections of such cells would then be desig- 

 nated embryologically as the anlage of striated 

 muscle, smooth muscle or heart muscle, as the 

 case might be. Yet it is not to be forgotten 

 that at this stage there might be really no 

 means of distinguishing between the several 

 different types of mesodermal cells. 



Limitization of potentialities in the indi- 

 vidual mesenchymal cells has apparently 

 reached a comparable stage just about the time 

 when the cells begin to wander upon the yoU?:- 

 sac of Fundulus. We have seen these cells as 



