ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 201 



Development of Vennetus.* — Prof. M. Salensky's studies on the 

 development of Yermetus commenced with the segmentation of the ovum, 

 which follows the plan common to molluscs. The small macromeres 

 that become formed have a mass of chromatic granules in the place of 

 ordinary nuclei ; this peculiarity seems to be due to the fact that these 

 nuclei multiply indirectly. The cells of the secondary endoderm are 

 the product of this division, and they likewise have masses of chromatic 

 granules in the place of nuclei. Later on, the form of the secondary 

 endoderm reproduces that of the embryo itself, and represents a mass of 

 cells swollen at its anterior extremity and flattened posteriorly. The 

 cells of the primitive endoderm — or, in other words, the macromeres 

 — are so arranged that their protoplasmic portion faces the secondary 

 endoderm ; and in sections it may be seen that the protoplasmic portion 

 of each macromere adheres to the cells of the secondary endoderm. The 

 ectoderm which invests the dorsal surface of the embryo is formed of 

 flattened cells ; in some ova the anterior marginal cell is divided into 

 two parts, one of which is at the edge of the blastopore, while the other 

 takes part in the invagination ; the latter represents the oesophageal cell. 

 The marginal cells, which form the hinder edge of the blastopore, extend, 

 primitively, in such a way that the blastopore represents a tube opening 

 into the primitive invagination, which is filled by cells of secondary 

 endoderm. The shell-gland appears early on the dorsal surface of the 

 embryo, at a time when there are no evidences of the mesoderm ; its 

 history offers no peculiarity. 



After sketching the early stages in the development of the organs of 

 the body, the author proceeds to describe the mesoderm, the first 

 appearance of which is late. This first appearance is very difficult to 

 recognize, since the mesoblast first consists of a few scattered cells ; 

 these, which are placed between the cells of the ectoderm and those of 

 the secondary endoderm, are difficult to detect, since the two primitive 

 layers are applied to one another, and their cells only differ in the 

 size of their nuclei. The mesoderm appears to be formed from the 

 ectoderm, the cells which give rise to it multiplying directly. The 

 rudiment of the mesoderm is arranged bilaterally, being formed, from 

 the first, of two plates, which may be compared to the mesodermal 

 stripes of Annelids. Independently of these there is, in Vermetus, a 

 third, unpaired, rudiment, which the author calls the pericardiac meso- 

 derm, but it does not appear till a later stage. The development of 

 the eyes is somewhat remarkable ; at the outer edges of the cephalic 

 plates there appear two hemispherical thickenings of the ectoderm, 

 and the eye is formed by a thickening of the cephalic plates with 

 delamination. Behind them the cephalic plates become invaginated 

 to form the cephalic ganglia ; as the cells of the cups multijdy, they 

 form several layers of cells, and exchange their cylindrical for poly- 

 hedral forms. The formation of the fibrillar substance is due to the 

 modification of the protoplasm of the cells of the invaginations ; as they 

 increase in size the cells become fibrillar. The eyes very early take 

 on the form of spherical vesicles, owing to the extension of their cavity ; 

 but they still remain intimately connected with the rudiments of the 

 cephalic ganglia, from which they do not become completely independent 

 till a comparatively late stage. There is no doubt that the wall of the 

 vesicle is converted into the retina. 



* Arch, de Biol., vi. (1887) pp. 655-759 (8 pis.). 



