(512 SUMMARY OF CURRENT RESEARCHES RELATING TO 



developed than the head-germs ; they extend as far as the hinder 

 end of the pharynx, and aro not connected with those of the head ; 

 posteriorly they terminate a little short of tho hinder end of the 

 larval body. They are at first separate, but later on they fuse, at first 

 in their hinder and then in their anterior portion. From their outer 

 margin the primitive kidneys are budded off, as simple rows of cells, 

 each of which forms a swelling at tho lateral margin, and still later 

 separates from the germ. The history of tho kidneys is given in 

 detail. 



The author prefaces his account of the formation of the body 

 of the adult by a statement of the views held by earlier writers ; ho 

 himself finds that tho primitive oesophagus, like tho primitive 

 ectoderm, is a provisional transitory structure, no signs of which are 

 to bo found in the adult ; instead thereof there is formed by the 

 union of the head and trunk-germs, and the invagination which takes 

 place at tho point where the primitive mouth was situated, tho 

 permanent oesophagus of the leech. In the Gnathobdellida the hind- 

 gut is not formed by the invagination of the primitive epidermis, but 

 as a growth from the trunk-germs. Of the larval body nothing 

 remains but the endoderm, and the body of the leech is formed by the 

 hind and trunk-germs, which grow around the epithelium of tho 

 midgut. 



After a critical review of what has been effected by his pre- 

 decessors, the author considers the question of the typical develop- 

 ment of tho Annulata ; the first important point is that the body is 

 wholly or partially built up of two pairs of germs, an anterior 

 and a posterior, which are, histologically, exactly similar, and which 

 grow around the mouth and enteron. In the clearest cases (Ne- 

 mcrtines, Leeches) there are four primitively common collective 

 structures which contain the rudiments of all the tissues and organs, 

 and from which the definite layers and tissues are only secondarily 

 differentiated. This " scheme " is essentially that of those whoso 

 ova are provided with a small quantity of nutrient yolk, and they 

 must be regarded as the typical, since all can be easily referred 

 to them. Though this might seem to show that the Nemertinea 

 and Hirudinea are closely allied, the author regards the latter 

 as true Annelids ; he finds an explanation in the fact that the 

 Nemertinea have the simplest, and the Hirudinea the most specialized 

 mode of development. 



In the Nemertinea all the tissues of the body (with the excejition 

 of a part of the enteric epithelium, and, perhaps, the lateral organs) 

 are typically formed from five germs, which (in Pilidiuni) arise as 

 hollow invaginations of the primitive ectoderm and grow around 

 the mouth. There is a late differentiation of the various parts in 

 the larvse. In the Polychasta and Oligochasta the four germs are 

 from the first differentiated into two parts, so that there are, so to 

 speak, eight germs ; these do not arise as hollow invaginations, but 

 as solid growths of the ectoderm, and the distinction between the 

 provisional and the permanent epidermis is no longer possible ; there 

 is a partial early differentiation. In the Leeches this early differen- 



