van Beneden on Bat 39 



Van Beneden regards the whole of the notochord, the whole 

 of the gut lining and the whole of the mesoderm, with the possible 

 exception of the endothelial lining of blood vessels, as being derived 

 from the primitive streak, i.e. by invagination. The primitive 

 streak is converted into the embryo by concrescence of its lips. 

 Even if the primitive streak does become converted into " embryo" 

 by concrescence, this does not necessarily prove that the original 

 blastopore existed along the dorsal surface of the ancestral 

 chordate. 



Because : 



(i) The primitive streak appears in the first instance as an 

 already closed blastopore. 



(ii) The grooving is I believe due to mechanical causes. 



I would interpret the bat condition (Fig. 21) as follows. 



All van Beneden's archenteric plate I regard as deuterogenetic. 

 His discarded " lecithophore " is protogenetic. His figures of bats 

 certainly do not support his contention that no notochord is 

 formed from the lecithophore, (v. Plate XVIII, fig. 7, 8). There 

 is evident thickening along the middle line due to proliferation 

 of nuclei at right angles to the surface. 



Gastrulation and Growth in Length in Birds. 



The segmented part of the ovum at the end of seven hours 

 after fertilisation consists of a little cap of cells occupying the 

 centre of the upper pole. These cells have been produced by the 

 segmentation of the germinal disc. They may be said to be in 

 cellular continuity with the so-called unsegmented yolk mass around 

 and below them. We may as well term this cap of cells the blasto- 

 derm. It is important to remember that this cap of cells is 

 continually being added to peripherally and perhaps beneath as 

 well. It should be noted that whenever a new segment is cut off 

 from the peripheral yolk mass the formation of such a new cell is 

 preceded by the division of a nucleus, and while one daughter 

 nucleus is contained in the new cell thus cut off and added to the 

 blastoderm, the other daughter nucleus remains behind in the 

 yolk mass, and after awhile it again divides with the result that 

 another cell is added to the blastoderm, and so on. Therefore 



