VASCULOGENESIS IN THE CAT 39 



lects the case of the umbilical vein, and is to that large degree 

 incomplete and partial. 



The youngest embryo which requires consideration here is 

 No. 550 of the Columbia Collection. The somites are forming 

 but as yet there is no corriplete intersomitic cleft; the coelom is 

 little more than a line of beginning cleavage; in a few places the 

 layers of mesoderm separate and narrow discontinuous clefts 

 appear. 



The parietal mesoderm is, in general, compact. It is separated 

 from the ectoderm by a space which must be considered in large 

 part due to shrinkage. In general, the outline of the mesoderm 

 is clear cut, but lateral to the neural plate and dorsal to the 

 coelom-clefts short protoplasmic processes extend from its periph- 

 eral cells into the hypectodermal space; a few are also to be 

 seen on the basal surface of the ectoderm. These are evidently 

 the fibers of Aurel v. Szily, 81 the interdermal cytodesmata of 

 Studnicka, 82 which collectively constitute the mesostroma of the 

 latter investigator. Subsequently, these processes form a felt 

 work between ectoderm and mesoderm, into which cells from the 

 mesoderm migrate. The latter process, the formation of the 

 somatopleuric mesenchyme, is in its inception in this embryo. 

 We may distinguish first a loosening of the cells of the mesoderm 

 characterized by an increased distance between the nuclei, the 

 appearance of intercellular clefts, and the projection of an oc- 

 casional nucleus beyond its fellows; second, the appearance of 

 cells, or more strictly energids, connected by broad protoplasmic 

 bridges with the mesoderm; in some cases the bridges are narrow, 

 the energids are distinctly pedunculated, and finally there are 

 a very few elements which are joined to the mesoderm by tenuous 

 cytodesmata only. 



In embryos in which the first intersomitic cleft is complete 

 (Columbia Collection Nos. 449. 554, 555) the mesenchyme has 

 increased in amount and the area involved is larger, now extend- 

 ing mesad along the parietal mesoderm as far as the somites 

 (fig. 1). The mesostroma is more abundant and in places forms 

 a feltwork of protoplasmic strands. The dorsal contour of the 



81 1903, Anat. Anz., Bd., 24, p. 417. 

 82 1911, Anat. Anz. Bd. 40, p. 33. 



