ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 357 



pleted. This phenomenon leads to the supposition that fertilization 

 takes place some time after the development of the polar cells, and 

 this view is supported by the fact that spermatozoa only appear in 

 the oviduct when that shortening is over. After the ova consist of 

 four parts the follicular cells begin to proliferate, and completely 

 surround the blastumeres, forming the chief mass of the embryonic 

 cells. In connection with the ovarian products there appears an 

 epithelial investment, derived from the thickened portion of the wall 

 of the respiratory cavity, which, later on, separates into an ectodermal 

 and a placental portion, and plays an important part in the formation 

 of the embryo. Cleavage goes on very slowly ; the oviduct continues 

 to shorten, and ends by converting the follicle into a saccular structure, 

 to the inner surface of which the cells become attached. As the 

 oviduct contracts, the follicle passes into the cavity of the ectodermal 

 portion of the epithelial outgrowth, and concludes by completely 

 filling it. After the separation of the epithelial outgrowth has com- 

 menced the wall of the respiratory cavity rises up around its base, in 

 the form of a fold, which later on embraces, with the placenta, the 

 whole of the embryo. 



The author points out that these results do not agree with those of 

 Todaro,* whose observations he discusses in some detail. 



The processes just described may be regarded as those of the fixst 

 developmental period, and we now come to a description of the external 

 form of the embryo in its different stages. The body is at first pyra- 

 midal in form, it then increases in length and breadth more than in 

 height, and so becomes flattened out ; a little later the placenta 

 becomes gradually separated from the embryo, until at last they are 

 only connected together by a short round stalk, which itself finally 

 disappears. Hand in hand with these changes the rudiments of the 

 organs begin to be laid down ; the heart appears very early, and the 

 pericardiac cavity is at first very large. The boundaries of the respi- 

 ratory cavity are, at first, very difficult to make out, as it is filled with 

 and lies in a mass of follicular cells. The internal cell-mass is 

 gradually absorbed, when the outer contours of the cavity become, of 

 course, better marked. Simultaneously with the appearance of the 

 egestive orifice we get the first signs of the musculature of the body. 

 This arises in the form of eight muscular bands which commence at 

 the upper end of the body and extend to about its middle. Somewhat 

 later we see the commencement of the ventral folds, which gradually 

 pass into the lower wall of the respiratory cavity. The enteric canal 

 is one of the last organs to appear, and the nerve-ganglion can only 

 be made out with dif&culty before it has assumed the form of a vesicle 

 and lost its relations to the outer cell-layers. The elaeoblast is not 

 developed early but grows rapidly so soon as it has begun to put in 

 an appearance. The history of these organs is carefully discussed, 

 and several points of disagreement with the accounts given by Todaro 

 appear in its course. 



In the latest periods of development we find a thickening of the 

 follicular wall, and the definite formation of the organs. In conse- 



* See this Journal, iii. (1883) p. 41. 



