Literary Notices. xxxiii 



alon. The azygos character of the prosencephalon obscures relations 

 somewhat. 



A second part of the paper treats of the preoral digestive tract 

 and the mesoderm of the head into which our limits will not permit us 

 to go. The author concludes, however, that in the ontogeny of the 

 Craniates there has been a retrograde metamorphosis in the ventral 

 region of the head, as a result of which the original cephalic part of 

 the digestive tract has been lost. The mouth of Craniates is not ter- 

 minal but is derived from a point ventrad and caudad of the front. 

 The hypophysis he regards as the original mouth, paleostoma, while 

 the functional mouth is a neostoma. 



The trabecular cranii in Ammocetes are related to the hypophysis 

 as the inferior maxillaries are to the mouth. 



The last section of the paper is devoted to the development of 

 the cranial nerves and as it requires fuller analysis than can now be 

 given it and forms but the introduction to the " study" next to appear, 

 it may be reviewed in connection with the latter. The clear and easy 

 style of the author and the elegant work of the publisher combine to 

 produce a very acceptable series. 



Embryology of Selachians. 1 



These observations form the principal part of the descriptive por- 

 tion of the author's work recently published in Russian on the devel- 

 opment of vertebrates. 



I. Origin of the Nerves. 



i . Separation of the nerve-tube from the ectoderm. The author 

 here adduces evidence to show that the whole central and peripheral 

 nervous system springs from a single proton (anlage). 



2. Primitive differentiation of the general proton of the periphe- 

 ral nervous system. By this proton we understand that embryonic 

 structure which gives rise to spinal ganglia, dorsal roots, and periphe- 

 ral nerve fibres. The differentiation of the ganglionic layer from the 

 dorsal wall of the nerve tube marks the first step in the development 

 of the peripheral system ; the dismemberment of this layer, the sec- 

 ond. In Raja there is no trace of this layer up to the closing of the 

 nerve-tube. Then a less compact arrangement of the walls of the 

 tube in the parietal region of the head forms the first step in this dif- 

 ferentiation. The epidermal groove is still well marked, but its walls 



^Iitroi'Hanow, P. Etude Embryogenique sur Selaciens, Arch, de Zool. 

 Exp. et Gen. 3d Series, I, 1893. 



