508 SUMilA-BY OF CURRENT RESEARCHES RELATING TO 



thick throughout, being more delicate where epiboly commenced and 

 in the region which, in time, will be the dorsal surface of the embryo, 

 than on that which is opposite to it. On the former there are two 

 prostomial pads which bound a prostomial invagination. The author 

 cannot agree with Goette in thinking that the mesodermal elements are 

 derived from the endoderm ; he regards the mesoblasts as forming a 

 continuous layer with the ectoderm, from the cells of which they 

 cannot be distinguished either by their origin or their position. 

 Other statements of the same author with regard to this layer are 

 then canvassed, and Salensky says that, a few hours later, the pro- 

 stomial buds disappear and the prostomium or blastopore becomes 

 a small pentagonal orifice ; the embryo is still spherical in form. 

 This portion of the paper concludes by insisting on the analogy which 

 exists between the formation of the mesoderm in N. cultrifera and the 

 genesis of the same layer as described by Kleinenberg for Lumhricus 

 trapezoides. 



On the fourth day we note a marked advance in the organization 

 of the embryo. The ectoderm still continues to be differenti- 

 ated and the groove which forms the rudiment of the ventral 

 ganglionic chain becomes apparent ; as yet, however, the medullary 

 plates merely consist of a single layer of cells, not yet sharply marked 

 off from the sides of the ectoderm. The two bands of mesodermal 

 cells now diverge much more than in the preceding stage, while they 

 still unite towards the hinder end of the embryo. Yet again, the 

 relation of the mesoderm to the ectoderm is still very close, and it is 

 difficult to determine whether it grows at the expense of the ectoderm 

 or not. In the fifth day we see various organs appear, such as the 

 setigerous sacs and the lateral and muscular plates ; the embryo now 

 becomes a little flattened on its dorsal side, and the ectoderm under- 

 goes delamination in the more anterior regions. The mesoderm, as 

 may be supposed, becomes considerably modified, but the endoderm 

 only consists of five large spheres. 



The sixth day is marked by the appearance of the first rudiments of 

 the feet, in the form of two pairs of tubercles, and herein hardly any 

 share is taken by the ectoderm ; the mesodermal segments become 

 indicated, and each is seen to consist of the rudiments of the seti- 

 gerous sacs, of the muscular, and of the lateral plates. On the next 

 day the dorsal cavity becomes very apparent and contains amoeboid 

 cells, which are most probably of ectodermal origin. On and after the 

 eighth day development proceeds very rapidly ; the embryo becomes 

 much larger, the cephalic region becomes separated by a crown of 

 cilia, and the eyes appear on its margins. A post-oral circlet of cilia 

 is developed, and the two become united on the back of the embryo. 

 Between the cephalic gauglia and their commissure we see a cellular 

 mass of triangular form, which is called the ocular plate, and it is 

 this which is, in all probability, converted into the hypodermis which 

 invests the eye. The conversion of the lateral plates into somato- 

 pleure and splanchnopleure is, as in Euaxes and Scorpio, carried on 

 independently in each segment. The other details of development 

 are given in full ; on the ninth day the embryo escapes. 



