ZOOLOaV AND BOTANY, MICROSCOPY, ETC. 539 



with the bilateral evagination of the primitive intestine, so that it 

 arises by delamination. This mode of development appears, hovrever, 

 to obtain only during the period in which the intestine is not yet 

 formed. "When the intestine becomes a closed tube, the mesoderm 

 is formed directly at the expense of a mass of indifferent cells ; in 

 other words, the endoderm and the mesoderm are intimately united 

 at the point where the embryo is growing forwards. The part of the 

 mesoderm formed by a process of delamination is, comparatively, very 

 small. At the edges of the blastoderm and of the blastopore — the 

 point where the embryo grows backwards — the three germinal layers 

 are closely united. 



The notochord is formed by the endoderm, and, like the mesoderm, 

 it grows forwards and backwards. Anteriorly the layers are so 

 closely united that the cord there appears to be solid, and the anterior 

 portion of the notochordal groove is, therefore, only feebly developed. 

 Posteriorly the delamination is very pronounced, and the groove is 

 wide and deep. The animal portion of the endoderm, or that which 

 gives rise to the notochord, is separated, on either side, by a narrow 

 but distinct cleft from the intestinal endoderm. Anteriorly the 

 notochordal and intestinal endoderm are completely fused, and directly 

 continuous with one another ; posteriorly they are for a long time 

 independent. This would seem to show that the mesoderm originally 

 arose, at the hinder extremity, by a bilateral delamination of the 

 primitive intestine, and this has, in the course of phylogenetic deve- 

 lopment, been replaced by a process of folding. 



The author applies his knowledge of the development of the 

 cartilaginous fishes to an explanation of the phenomena of the de- 

 velopment of the notochord and mesoderm in birds ; and comes, in 

 conclusion, to the result that the phenomena observed in the mero- 

 blastic ova of cartilaginous fishes amply demonstrate the truth that 

 there is no well-marked division between mesoblast and mesenchyme, 

 as has been insisted on by the brothers Hertwig. In these fishes 

 only a small part of the median germinal layer is formed by the 

 bilateral evagination of the primitive intestine — the mesoblast of 

 the Hertwigs. The greater part of the layer would be, for the 

 Hertwigs, mesenchyme. As a matter of fact, the two unite so early 

 that it is impossible to say which had the earlier origin. The cells 

 of the part which arise as mesenchyme have the same epithelial 

 appearance as those which are formed by the evagination of the 

 primitive intestine. The body-cavity >—enterocoele — which was at 

 first found only in the part of the median layer which arose by 

 delamination, soon extends into the region which, from its mode of 

 origin, should be called mesenchymal. 



Intra-cellular Digestion in the Germinal Membrane of Verte- 

 brates.* — J. Kollmann commences with an account of his observations 

 on the cells of the endoblast in the lizard ; these cells vary consider- 

 ably in size, and in thtir protoplasmic contents one finds spheres 

 which are to all appearance of a fatty nature, and which are also 



• Eecueil Zool. Buissp, i. (1884) pp. 259-90 (1 pi.). 



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