ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 



89 



by tte general body-cavity is described. The dorso-ventral muscles are 

 maiuly derived from tbe elements of the disintegrated partitions. The 

 circular and longitudinal muscles and a large portion of the connective 

 tissue are developed at the expense of the parietal layer of the mesoderm. 

 The connective tissue which forms the sheath and support of the nervous 

 system is formed from ectodermic elements, which at a less advanced phase 

 appear as a thick mass in the anterior portion of the embryo. All the rest 

 arises from the mesodermic elements which result from the disintegration 

 of the somite walls. 



2. The alimentary canal. — In regard to this point Hoffmann and Whit- 

 man have arrived at opposite results, the former deriving the digestive 

 tube from the mesoderm, the latter from the primitive endoderm. Nusbaum 

 has shown the correctness of Whitman's conclusion. Even the epithelium 

 lining the cavity of the proboscis is of endodermic origin, and the posterior 

 intestine has a similar history. 



3. The nepJiridia appear as accumulations of mesoderm cells, as differ- 

 entiations of the parietal sheath, in the anterior angle of each somite, 

 abutting directly on the anterior septum. They appear in all the segments, 

 but are reduced in the most posterior. Certain large reproductive cells are 

 locally associated with their development. The appearance of the cavity 

 and subsequent stages are carefully described. 



4. The reproductive organs. — After giving a full account of the anatomy 

 and histology of the reproductive organs, the author describes their 

 development. Eight large endoderm cells at the posterior pole of the 

 embryo (Whitman's "neuroblasts") multiply rapidly and extend forward 

 until finally a pair are found in each segment. As the separate somites 

 appear, the sex-cells are found disposed at the base of the septa. They 

 multiply rapidly and form (1) a pair of cellular masses at the boundary 

 between proboscis and mid-gut — the ovaries, and (2) six groups of sperm 

 cells separated by lateral diverticula of the mid-gut. At the posterior 

 portion of the proboscis, isolated free-cells are formed, probably reduced 

 yolk-forming cells, as in Amphilina-Planaria for instance. The young 

 ovaries are solid rounded masses with characteristic cells, they are sur- 

 rounded by a mesodermic endothelium. They become associated with a 

 pair of nephridial rudiments, the expanded ends of which embrace the 

 ovaries. The nephridium forms the oviduct, and external elements add 

 muscle-fibres and outer membrane. Bound each group of sperm cells a 

 mesoderm endothelium is formed, and the delicate transverse canals con- 

 necting testes and vasa deferentia are formed from this envelope. The vasa 

 deferentia represent a pair of modified nephridia. Further details of the 

 development are given, and the free disposition of all the organs within 

 the body-cavity is compared with what is permanent in Gunda segmentata 

 and Amphilina-Planaria. 



5. The nervous system. — In regard to the development of the nervous 

 system in Clepsine, there has been a good deal of haziness. Nusbaum has 

 shown its origin from a ventral ectodermic thickening forming the ventral 

 cord, and an anterior dorsal forming the brain. The insinking of the cord 

 is carefully described. At an early period isolated endodermic cells apply 

 themselves to the surface of the cord, forming a sheath of flat cells — the 

 internal neurilemma, but also sending prolongations between the elements 

 of the nerve-cord and dividing it into three portions. These penetrating 

 elements form a delicate network separating the cellular portion of the 

 chain from the fibrillar. The author also notes the temporary connection 

 of ventral blood-vessel and nerve-cord. Anteriorly the wall of the 

 ventral vessel is seen to be prolonged into two lateral plates, which unite 



