18 SUMMARY OF CURRENT RESEARCHES RELATING TO 



Development of Cranial Nerves and Ganglia in Trout.* — Dr. N". 

 Goronowitsch adheres to a conclusion which stands opposed to most of 

 the work of the last quarter of a century as regards the development 

 of cranial nerves and ganglia. The " neural ridges " or outgrowths 

 from the central nervous system, which Balfour and Marshall regarded 

 as the origins of cranial nerves and ganglia, are said by Goronowitsch 

 to be resolved into the surrounding mesoblast and not to have any- 

 thing to do with the ganglia. These arise from Froriep's Kiemen- 

 spaltorganen, Beard's " branchial sense-organs." Like Sedgwick, who 

 has described the development of nerve-fibre in situ in the mesodermic 

 reticulum, Garonowitsch does not hesitate to call the nerve-forming 

 tissue mesodermic, — which is an interesting return to the position of 

 Remak. It is certainly " upsetting " to be informed — and with no lack 

 of details and figures — that nerve-fibres are not processes of ganglion- 

 cells, and that the neural ridges have nothing to do with the develop- 

 ment of the cranial ganglia. 



Metamerism of the Elasmobranch Head.f — A. N. Sewertzoff makes 

 a contribution to the subject, dealing in particular with the head of 

 Torpedo. It is not possible for us here to do more than indicate his 

 general conclusion. The head-region of Batoids and Selachoids is 

 developed on similar lines ; there are two anterior (premandibular and 

 mandibular) segments associated with the visceral arches, and behind 

 these a series of head-somites homologous with the protovertebras of 

 the trunk. In Pristiurus and Scyllium there are 7 (3-9 of van Wijhe), 

 in Acanthias 8 (3-10 of Hoffmann and Neal), in Torpedo 11 (3-13 of 

 Sewertzoff). It seems that in the more specialised forms an increasing 

 number of anterior trunk-segments are appropriated to form part of the 

 head. On this view, Pristiurus is primitive, and Torpedo specialised. 

 The author compares his conclusions as to Elasmobranchs with the facts 

 known in regard to Amphioxus and Cyclostomata, and gives a hypo- 

 thetical description of the state of affairs in the original Vertebrates. 



Metamerism of Head in Petromyzon planeri.+ — Herr N. K. Koltzotf 

 has studied this subject in an extended series of embryos, and publishes 

 a preliminary account of his results. The first trace of segmentation 

 appears at the stage where the fourth and fifth somites are differentiated. 

 Though the first two somites, the premandibular and the mandibular, 

 differ in some respects from the other somites, there is no reason to 

 believe that they are anything but true somites, especially as the evi- 

 dence is against von Kupffer's suggestion that the premandibular somite is 

 a gill-pocket. This somite is always small, and does not acquire a cavity 

 until the embryo is 3-1 mm. in length. The cells seem to form the 

 oculomotor musculature. As in Selachians, it is difficult to distinguish 

 between the second somite and the mandibular arch. The walls of the 

 former give rise to the musculus obliquus superior, and possibly, though 

 not probably, to the " prolongation of the velar muscles " (Hatschek). 

 The gill-pockets do not show the same metamerism as the myotomes ; 



* Nouv. Mem. Soc. Imp. Nat. Moscou, xvi. (1898, received 1899) pp. 1-55 

 (3 pis.). See Nature, Ixi. (1899) pp. 158-9. 



t Bull. Soc. Imp. Nat. Moscou, 1898 (published 1S99) pp. 393-445 (4 pis.). 

 t Anat. Anzeie., xvi. (1899) pp. 510 23 (3 figs.). 



