ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 397 



Cejmalopods tho existence of three germinal layers, the outer columnar 

 epiblast, the intermediate polygonal mesoblast cells, and the inner 

 spindle-shaped hypoblast cells. The evidence as to the yolk-membrane 

 being the hypoblast is further discussed, and some objections to this 

 view are noted. The fact that it does not take any part in the formation 

 of the digestive tract, which consists only of fore-gut and hind-gut, may 

 be explained by the quantity of food-yolk to be absorbed being so large 

 that other structures are completed before it is all absorbed. 



Shell-growth in Cephalopoda.* — Mr. F. A. Bather returns to this 

 subject.^ His conclusions are that the whole of the true shell, and the 

 whole of the sheath are first formed in chitinous membranes, secreted by 

 the visceral hump and mantle respectively ; these become calcified by the 

 deposition in their interstices of arragonite and calcite respectively ; 

 there is no intussusception, except of lime, and that is probably a 

 physical process. Secretion of chitin continues after growth ceases, 

 and may be accelerated in phylogeny. The rate at which lime is 

 deposited is independent of the animal, and hence extent of calcification 

 varies inversely as rapidity of chitin secretion. 



Systematic Arrangement of Cranchia.J — Dr. J. Brock, referring to 

 Mr. Hoyle's report on the ' Challenger ' Cephalopoda, in which Cranchia 

 Reinhardtii of Brock is regarded as not identical with the type specimen 

 of Steenstrup, which is preserved in the Copenhagen Museum, enters into 

 some details as to his specimen ; the differences do not appear to him to 

 justify the formation of a new species. 



y. Gastropoda. 



Spermatogenesis in Aplysia.§ — M. E. Robert has investigated the 

 development of spermatozoa in Aplysia depilans and A. fasciata, where 

 he finds two different processes. In the first, which appears to be the 

 more normal and frequent, the nuclei of the spermatoblasts divide into a 

 certain number of parts ; these elongate and take on the form of spiral 

 filaments, while the nucleus continues to grow ; the filaments, which are 

 the heads of the spermatozoa, separate from one another. As the nucleus 

 grows it seems to absorb the surrounding protoplasm, which becomes 

 reduced to a delicate zone. At maturity the protoplasm of the sperma- 

 toblast is all absorbed by the nucleus. The nuclein is divided into a 

 number of spiral filaments, each of which is the head of a spermatozoon. 

 The nucleus bursts, and the heads of the spermatozoa escape in the 

 form of large Vibrios. The author believes that the tails of these 

 elements are formed by the elongation of a portion of the spermato- 

 blastic protoplasm which is carried away by the cephalic filament of 

 chromatin. 



In the second mode of development the spermatoblastic cell gives 

 rise to only one spermatozoon. The nucleus, instead of dividing into 

 distinct masses, elongates at one extremity, and takes on the form of a 

 strongly curved rod; its other end elongates, and it becomes of an 

 elongated fusiform shape. Finally the middle becomes more delicate, 

 and elongates in its turn. This elongation is not, however, straight, but 



* Ann. and Mag. Nat. Hist., i. (1888) pp. 298-310. 

 t See this Journal, ante, p. 200. 

 t Nacbr. K. Gesell. Wiss. Gottingen, 1887, pp. 320-2. 

 § Comptes Rendus, cvi. (1888) pp. 422-5. 



