VASCULOGENESIS IN THE CAT 47 



The thickenings soon come to project Trom the ventral surface 

 of the mesoderm, and begin to separate from it. At about the 

 time when the first somites are forming and before definite inter- 

 somitic clefts have appeared, the migration of energids becomes 

 extensive and desquamation is also well marked. The uncleft 

 mesoderm, the visceral layer of the incipient coelom, and the in- 

 termediate cell mass are all affected (fig. 20). Many strands 

 of mesostroma cross the hyperentodermal space and among these 

 the mesenchyme cells are scattered, singly or in flat plate-hke 

 groups. They show every degree of connection with the com- 

 pact mesoderm, the plates in particular being often broadly con- 

 nected. It is a repetition on a larger scale of the processes ob- 

 served in the somatopleure, earUer in time, and more active. As 

 yet no endothelium or blood cells have been formed. 



In the model of the embryo of two somites (figs. 21 to 23) the 

 distribution of the tissue in the hjTjerentodermal space is shown 

 in relation to the compact mesoderm (white) — the mesenchjone 

 is green, endothelium red, and the intermediate forms of tissue 

 (vasofactive cells) yellow. Cephalad the line of the heart, ven- 

 tral aorta and first arch is occupied by scattered mesen hyme cells 

 and ceU groups. There are also a few isolated mesenchyme ele- 

 ments of more mesal position. The line of the aorta is vacant 

 to within a short distance of the first somite. Here scattered 

 cells and plates are present. Ventral to the somites and imme- 

 diately caudal to them an irregular continuous plate or strand 

 has appeared. As yet this is connected with the more lateral 

 mesenchyme in which the omphalomesenteric plexus is forming 

 only at one point. There are, however, numerous patches of 

 mesenchyme scattered in the hiterval. Caudad in the region of 

 the primitive streak there is no axial mesenchyme. There is 

 much mesenchyme, associated with vasofactive cells and endo- 

 theUum, along the lateral margin of the coelom and extending 

 mesad below the visceral mesoderm. This is most abundant 

 caudad of the heart and the transverse plane of the somites, ap- 

 proximately in the region which Schwink*^ regarded as the point 

 from which endothelium sprouts. 



86 1891, Morph. Jahr., Bd. 17, p. 288. 



