ZOOLOGY AND BOTANY, MICROSCOPY. ETC. 871 



whicli forms a specialized independent layer, and which we may, at a 

 later stage, regard as the primitive endoderm ; the blastoderm becomes 

 differentiated into an ectoderm, which forms a rounded pliite one layer 

 thick in its centre, and formed of several rows of cells at the periphery ; 

 these cells are formed by delamination ; the deeper layers of the edge of 

 the blastoderm form part of the mesoderm. 



The author concludes with carrying further the modifications of the 

 germinal layers, and takes the oppc.rtnnity of discussing the views of 

 those who have preceded bim. Other parts of the mesoderm are formed 

 by the ectoderm in different parts of the body ; as at the periphery of 

 the eye, the region of the siphon, where it gives rise, by proliferation, 

 to a mass in which the muscles of the siphon and the visceral ganglion 

 are, later on, differentiated, the cephalic lobes, and the extremity of the 

 brachial folds. It will be seen that the character of these secondary 

 delaminations is to be isolated and partial, and this secondary prolifera- 

 tion of the ectoderm produces very different tissues, for the masses to 

 which it gives rise are sometimes muscular and sometimes nervous. The 

 author does not think there can be any doubt as to the justice of regarding 

 the perivitelline membrane as the primitive endoderm. 



7. Gastropoda. 



Double Forms of Spermatozoa.* — Dr. E. Koehler gives a descrip- 

 tion of the two forms of spermatozoa found in Murex hrandaris and 

 M. trunculus. It is pointed out that in the Pulmonate Gastropods (Anon, 

 Helix) the mother-cells of the spermatic products, or spermatogonia, 

 arise from nuclei which are scattered irregularly in a layer of protoplasm 

 which lines the inner surface of the testicular tubes. These nuclei with 

 the protoplasm correspond to the regular embryonic epithelium of these 

 tubes, and represent the primordial sexual cells. It is probable that 

 these nuclei also give rise directly to the special elements which have 

 been called basal cells. 



In Murex, the nuclei, which are arranged in a similar manner, give 

 rise to two very distinct categories of elements ; some are large cells 

 with definite contours which are the mother-cells of the vermiform 

 spermatozoa, while others are smaller, have no membrane, and give rise 

 to the filiform spermatozoa ; these last undergo the changes usual in 

 spermatogenesis. The mother- cells of the vermiform spermatozoa 

 inclose only a single nucleus which will, later on, break up to form the 

 large multinucleated cells ; they do not undergo the repeated divisions 

 which cliaracterize the development of ordinary spermatozoa. The 

 substance of one of the nuclei is converted into a bundle of fibrils, one 

 end of which will give rise to a tuft of cilia, while the other will form 

 the cephalic extremity of the vermiform spermatozoon. During its 

 development the nuclei largely disappear in the cellular protoplasm, but 

 their remains will form the colourable granulations which the proto- 

 plasm of their spermatozoa contains in their adult state. These have no 

 distinct central filament, except in the cephalic region. Although the 

 organization of the spermatozoa is similar in the two species, those of 

 Murex hrandaris are immobile, and of M. trunculus very active, The 

 vermiform spermatozoa of the Prosobranch Molluscs are not adapted to 

 any definite function ; as their early history shows, they have the 



* Rec. Zool. Suisse, v. (1888) pp. 101-50 (2 pis.). 



