812 SUMMARY OF CURRENT RESEARCHES RELATING TO 



which invests the Berve-cord, and appear in the form of a mass of 

 similar cells ; these cells subsequently become differentiated into 

 ovular and epithelial cells ; and, later on, the ovisac becomes hollow, 

 and, changing its position, makes its way towards the epidermis. 

 The male reproductive organs have an analogous structure to the 

 female. 



The ova are exceedingly small and very transparent, and observa- 

 tions on the early stages are somewhat incomplete ; towards the end 

 of the period of segmentation the macromeres and micromeres cease 

 to have a different appearance, and we get a well-marked archiblastula, 

 such as has been seen in other Nemertines. The mesoderm appears 

 directly after the formation of the blastula, and calls to mind the 

 phenomena seen in Lineus lacteus. The blastopore is circular in 

 form, but the gastrula is not radially symmetrical ; the former closes 

 without leaving any sign either of mouth or anus. The external 

 form of the larva now undergoes some changes, and the proboscis 

 begins to be developed from the ectoderm. This precocious develop - 

 ment of the proboscis and its sheath agrees in all that is known as to 

 the development of this organ in Pilidium. The endoderm now 

 consists of cells which are provided with prolongations which pene- 

 trate into the digestive cavity, interlace, and completely fill it. 

 Later on the endodermal cells take on again the form of an epithelial 

 layer investing a large digestive cavity. 



In later stages it is found that the epidermis is completely 

 developed from the ectoderm, and during its development undergoes 

 only insignificant changes ; the cells elongate and become cylindrical 

 and, later, divide into two layers; those of the outer are ciliated. 

 The cephalic gland is seen to be nothing but a well-developed mass 

 of ectodermic glandules ; it later divides into a ventral and a dorsal 

 portion. 



The nervous system appears as two thickenings of the ectoderm 

 which make their way into the body but are still connected with the 

 outer germinal layer ; these are the first signs of the cephalic ganglia. 

 They very soon separate from the ectoderm, and are connected with 

 one another by a commissure which corresponds to the ventral com- 

 missure of the adult. In very young embryos the posterior ex- 

 tremities of the ganglia begin to elongate, and we have here the first 

 signs of the lateral nerves ; they grow from before backwards, and 

 soon reach the hinder end of the body. In structure they closely 

 resemble the ganglia. 



It seems safe to conclude that in Monopora vivipara the cephalic 

 ganglia and the ventral commissure arise from two ectodermal 

 thickeaings at the anterior end of the embryo ; that the dorsal com- 

 missure is probably derived from the union of the dorsal lobes above 

 the proboscis ; and that the lateral nerves appear as prolongations of 

 the cephalic ganglia. The author then discusses the homology of the 

 nervous system of Nemertines with that of Annelids, and, after con- 

 sidering the results of various observers, gives his own views in the 

 following comparative table : — 



