ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 457 



Earliest Blood-vessels in Man.* — J. L. Bremer finds that in the 

 human embryo the earliest blood-vessels arise separately in the yolk-sac 

 and in the body-stalk, by multiple primordia. The prirnordia in the 

 body-stalk (and perhaps also in the yolk-sac) are funnel-shaped ingrowths 

 of the surface mesothelium, which is present as a definite layer only on 

 the two areas mentioned. By partial fusion of the walls of an ingrowth 

 a portion of the ccelom, still bordered by mesothelium, may be cut off as 

 a separate cavity, lying deep within the substance of the body-stalk. 



The endothelium seems to arise either (a) by a delamination from 

 the walls of such a detached portion of the ccelom, or (b) by direct 

 extension, in the form of an angioblast cord, from the mesothehal 

 ingrowth. From the endothelium, by whatever method developed, 

 further extension is by means of the angioblast cords, which grow 

 apparently through the surrounding mesoderm. 



The angioblast cords are apparently solid cords of cells, connected 

 end to end or in small groups, running between the processes of the 

 surrounding mesenchymal cells, when these are present, often touching 

 them, without however actually fusing with them. The cords tend to 

 form nets by anastomosis of larger mesh than the mesenchymal net, and 

 angiocysts by vacuolization wherever space is given. They are usually 

 sharply defined from the surrounding tissue, and may show an extra- 

 intimal space. 



True blood-islands may occasionally arise by the multiplication of 

 the cells of the mesothelial ingrowths, or scattered blood-corpuscles may 

 arise singly within these ingrowths. Extension within the limit of the 

 areas covered by the mesothelium is achieved by confluence of the 

 detached portions of the ccelom, or union of the cords ; the result is a 

 net comprising the various vascular units. Extension into the chorion, 

 where the mesothelial layer is absent in the early stages, appears to be 

 by direct centrifugal growth of the angioblast cords, without the addition 

 of new elements from the surrounding mesenchyma. The possibility 

 that similar, but later, ingrowths from the mesothelium of the intra- 

 embryonic ccelom may give rise to intra-embryonic vessels, should be 

 borne in mind in the study of such vessels, whether haemal or lymphatic. 



Development of Posterior Lymph-heart in Chick.f — Randolph 

 West finds that the lymphatic plexus which later enters into the forma- 

 tion of the posterior lymph-heart arises by the confluence of independent 

 mesenchymal spaces which connect secondarily with the veins. These 

 spaces are bounded at first by mesenchymal cells which later become 

 flattened to form an endothelium. Both in the endothelial lymphatic 

 walls and the adjacent mesenchyme there is an active haeroopoesis, the 

 products of which reach the general circulation via the lymphatic plexus. 

 Attention may be drawn to three very fine coloured reconstructions of 

 the caudal vessels of a chick. 



Development of Thymus in Pig.J— J. A. Badertscher finds that 

 the thymus of the pig has an ectodermal-endodermal origin. The 



* Amer. Journ. Anat., xvi. (1914) pp. 447-75 (11 figs.). 

 t Amer. Journ. Anat., svii. (1915) pp. 403-36 (14 figs.). 

 % Amer. Journ. Anat., xvii. (1915) pp. 317-36 (2 pis.). 



