40 VASCULOGENESIS IN THE CAT 



mesoderm is irregular: many of its elements project into the hy- 

 pectodermal space, some broadly, some, narrowly pedunculate, 

 others connected to the underlying mesoderm only by fine cy- 

 todesmata. The cytoplasm of these energids' is very scanty; 

 the nuclei are large, vesicular, with one or sometimes a second, 

 large spherical karyosome. In view of their scanty cytoplasm 

 and the protoplasmic nature of the mesostroma there is no serious 

 error in describing the process as a wandering .of the nuclei, as 

 Mall*'' does in the case of a similar phenomenon in the endocar- 

 dium. In both cases mesenchyme is forming, here from mesoderm, 

 there from endothelium; the latter therefore has not with its 

 change of form lost the potentialities of mesoderm in this respect. 



An embryo*^ (Columbia Collection No. 539), in which two 

 complete intersomitic clefts are present shows a marked increase 

 in the hypectodermal mesenchyme. Its distribution is shown 

 in figures 2 to 4. In this case as in the following models, the com- 

 pact mesoderm is reconstructed in white, in contradistinction to 

 the mesenchyme which is green. The surface appears more 

 uniform than is actually the case, for it was found impossible 

 to retain the innumerable small strands of mesostroma and all 

 the minute projections and irregularities that characterize the 

 contour of the mesoderm in section (fig. 5). The model, therefore, 

 shows only the topography of the mesenchyme. This is most 

 abundant in the head region where the axial mesoderm is less 

 compact than elsewhere, and along the lateral margin of the 

 coelom in the plane of the somites and for some distance caudad. 

 To a less degree, mesenchyme has formed from the parietal 

 mesoderm and the doi'sum of the somites. 



In section, the somites have regular contours, but elsewhere 

 the mesoderm gives rise to innumerable strands of mesostroma 

 joining it to ectoderm. The mesenchyme elements have in- 

 creased, and in addition to individually projecting cells, which 

 show every degree of emergence from the mesoderm, larger proc- 

 esses of this layer extend into the hypectodermal space. These 



«= 1912, Amer. .Jour. Aiuit., vol. 13, p. 249. 



'* I desire to cxi)ress my appreciation and thanks to Prof. J. II. .McGregor for 

 the gift of this beautifully preserved embryo. 



