82 



of La Valette), consisting of the liiglily refracting vesicles 

 which form the greater part of the vitellus, and sometimes 

 called " vitellus of nutrition/^ These two parts separate 

 when the blastoderm is in process of formation. The young 

 egg has no cell-membrane, as proved by its amoeboid move- 

 ments and aspect. In the oviduct it gains a covering, which 

 is really a chorion, and not a vitelline membrane. An exo- 

 chorion forms in some Crustacea. In Chondr acanthus there 

 is a micropyle in the chorion for the passage of spermatozoids, 

 whilst in the Amphipods there is none. In the various 

 groups of Crustacea the cells of the blastoderm give rise to a 

 structureless membrane formed by secretion, which is the 

 primary embryonic membrane — the Larvenhaut of German 

 writers. Some crustaceans present yelk cleavage, others do 

 not; and even in the same genus (Gammarus) some species 

 show the phenomenon, others offer no trace of it. The 

 blastoderm does not result, as generally asserted, from a 

 change of the yelk- masses at the periphery of the ovum after 

 cleavage into blastodermic cells, whilst others fuse together 

 at the centre to form a mass of nutritive matter; but this 

 phenomenon results from the accumulation of the deutoplasm 

 at the centre of the egg, whilst the protoplasm separating 

 from this, and taking with it the nuclei of the cleavage masses, 

 appears at the periphery to form the blastodermic cells. Gam- 

 marus (marine ^orms) , Dermophilus, Chondracanthus, and Co- 

 pepoda present the separation of deutoplasm and protoplasm 

 immediately after complete cleavage of the yelk. Anchorella, 

 Clavella, Caligus, &c., present no yelk cleavage, but the 

 deuto- and proto-plasm separate directly after fecundation. 

 The blastodermic cells develop ])y division from the original 

 egg-cell thus separated from the deutoplasm, and enclose and 

 spread over the whole surface of the yelk. In a third type, 

 realised in the Gammarus of fresh waters, whilst the deuto- 

 plasm is not included in the raAiltiplication of the egg-cells, 

 so as to constitute true yelk cleavage, yet the multiplication 

 does not take place from a point at the periphery, but a 

 number of cells arc produced simultaneously, which pass 

 from within to various parts of the periphery. In this 

 memoir many interesting facts are detailed, such as the effect 

 of heat and light in promoting cleavage, and the existence of 

 parasitic Amphipods forming the new genus Dermophilus, 

 living on the Lophius piscator'ms. 



In Memoir No. 2 Dr. Van Bencden gives the results of 

 inquiry into the development of the Isopod Asellus aqua- 

 ticus, so common in ponds. He criticises the papers 

 lately published by G. O. Sars and by Dohrn. The egg on 



