ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 709 



closed, but remains as anus. Tlie teloblast lies in front of or dorsal to 

 the blastopore, tbe reverse of its position when neurenteric canal or 

 corresponding strand is formed. The teloblast is not the primitive streak, 

 but corresponds only to its posterior end, and possibly also to the pole- 

 cells of the mesoderm described by Hatschek in AmpMoxiis. 



The spinal cord and notochord are formed together from a simul- 

 taneous activity of both germinal layers, resulting in the development of 

 a massive double keel before alluded to. The separation and further 

 development of both axial organs are then described. 



The mesoderm appears differently in head and trunk. In the former, 

 ccelomic diverticula are formed as in AmpMoxiis. In the latter the two 

 external layers of reserve yolk-cells form first the dorsal blocks, and then 

 the lateral plates, the somatic before the splanchnic. The coelomic cleft 

 appears at the same time as fore-kidney and heart. The pronephric duct 

 is ectodermic. 



The rest of the memoir is mainly devoted to a description of the 

 development of the nerves. The optic nerve is an exception to the usual 

 rule in this, that its ganglion appears much earlier than all the rest, and 

 arises not from a peripheral portion of the epidermis, but from the median 

 keel — that is, from the common origin for brain, spinal cord, and this pair 

 of ganglia. The branchial nerves form a second series, and the dorsal 

 spinal nerves a third. 



An anterior endodermic diverticulum is protruded between notochord 

 and epidermis dorsal wards to the brain. It forms a narrow median por- 

 tion and two lateral pockets. The former represents the well-known 

 diverticulum between hypophysis and notochord ; the latter form the 

 paired pre-oral head-cavities, which Kupffer regards as homologous with 

 the anterior endodermic diverticula in Amphioxus. 



Partial Impregnation.* — Prof. A. Weismann and Mr. C. Ischikawa 

 report that on examining the sexual cells of certain species of Moina 

 they found to their astonishment that those in which four segmental 

 cells were already present still contained a sperm-cell. This was found, 

 by further observation, to be a case of partial impregnation, only one of 

 the first four segmental cells and not the entire egg-cell becoming united 

 with the sperm-cell. In Moina paradoxa a spermatozoon penetrates 

 into the region of the vegetative pole of the egg, immediately after its 

 extrusion into the brood-chamber, where the egg is a naked sausage- 

 shaped mass. The vitelline membrane then becomes formed and prevents 

 the entrance of a second spermatozoon. The two polar bodies become 

 constricted off, and the nucleus of the ovum migrates to the centre of the 

 egg. The first two segmental cells appear, the sperm-cell always lying 

 in the neighbourhood of the one which is nearest the vegetative pole, 

 without, however, becoming united with it. The four-cell stage follows, 

 and the sperm-cell is now seen to exhibit amoeboid movements, and to 

 approach a segmental cell ; fusion then follows and in the next following 

 stage, that of eight segmental cells, no sperm-cell can any longer be 

 seen in the egg. 



Since making these observations "j" the authors have found that, " in 

 spite of the entire accuracy of our facts, we were mistaken as to the 



* Ber. Naturf. Gesell. Freiburg i. B., iv. (1888) p. 51. See Nature, xxxviii. 

 (1888) p. 329. 



t Translated (from a proof) in Nature, xxxviii. (1888) pp. 329-30. 

 1888. 3 C 



