409 



their extreme fecundity. In this case the egg is clearly not 

 a cell, but an aggregate of cells. 



In the preceding paragraphs avc have the summary of Dr. 

 Van Beneden's view of the structure of the animal ovum — 

 his main point being the recognition of this Deutoplasm, 

 which his detailed observations and figures, comprising 

 minutes tudies of the ovogeny of Trematods, Cestoids, Tur- 

 bellaria, Nematods, Rotifers, Copepods, Iso^jods, Amphipods, 

 Mysids, Mammals, and Birds, clearly establish. 



The mass of important observations contained in this part 

 of the work well deserve careful perusal. 



The tubular structure of the Mammalian ovary has been 

 observed by the author, confirming Pfliiger ; but he diflers 

 from that author as to the mode of origin of the cell-egg in 

 this case. Dr. Van Beneden has established in a variety of 

 cases, and in that of ISfammals (and man himself) , that the 

 cell-egg originates in a mass of protoplasm, contained in the 

 ovigerous tubes in the case of Mammals. Several distinct 

 nuclei arising by free development, these enlarge, and each 

 developes a nucleolus ; they ai-e, in fact, germinal spot and 

 germinal vesicle respectively ; around each germinal vesicle 

 the protoplasm then segregates, and the cell-egg is complete. 

 Its vitelline membrane, bounding the segregated protoplasm, 

 develops later, and in most cases after the addition of the 

 dentoplasm, or vitellogenous matter, as it used to be called. 



The very perplexing question of the disappearance of the 

 germinal vesicle at the time of or after fertilization in some 

 eggs — e.g., those of Mammifers, Birds, some Crustacea, and 

 some Mollusca — is discussed by Dr. Van Beneden. It dis- 

 ajDpears also in marine Annelids, according to Claparede and 

 Mecznikow ; and in Batrachia, according to Strieker, who, 

 as well as other workers, has very carefully searched for it in 

 the frog's and toad's ovum at the time of fertilization, but in 

 vain. On the other hand, we have to put against this disap- 

 pearance in the cases cited the observation of its division into 

 tAvo, causing the first yelk-furrow, as observed by Miiller in 

 Entoconcha, by Leydig in Rotifers, by Leuckart in Puppi- 

 para, by Mecznikow in Cecidomyia and Aphides, by Pagen- 

 stecker in Oxyuris, by Kefer stein in marine Planarm, by 

 Bessels in Lumbricus and Leeches, by Van Beneden and 

 Bessels in many Crustacea, by Van Beneden in certain 

 Trematods figured in this Avork. We particularly refer the 

 English reader to an important paper by Professor Huxley 

 on the ovum of Pyrosoma, to which we are sorry Dr. Van 

 Beneden has not had his attention directed (' Annals of 

 Natural History,' Third Series, vol. 5, 1860, p. 29). Certain 



