ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 619 



cavity, whence the enteric vessels are derived. It is important to note 

 that in the lower Annelids (e. g. Protodrilus leuckarti) there is 

 permanently some snch arrangement of the blood- vascular system, 

 while, further, we are shown that at first the blood-vessels have 

 nothing to do with, but are completely independent of, the lymphatic 

 spaces (coelom, &c). 



In Branchiobclella the ova, like their parent, are to be found 

 in the gill-laruellaa of the crayfish ; they are of some size and 

 are covered by a hard shell, produced posteriorly into a small stalk of 

 attachment. There is very early a difference between the appear- 

 ance of the dorsal and ventral surface, and cleavage obtains much 

 more rapidly among the cells of the latter. The endoderm and 

 mesoderm are formed by the division of the macromeres, which 

 constantly extends from behind forwards ; no primitive mesoblasts 

 were detected. Soon after the formation of the layers a small 

 depression appears on the dorsal surface, the significance of which 

 could not be made out, though it was possibly the rudiment of the 

 supra-oesophageal ganglion. Soon after the appearance of this de- 

 pression there appears on the ventral surface a large groove — the 

 neural groove ; this is pyriform in shape, the broader end being 

 posterior, and the narrower anterior end having a T-shajied enlarge- 

 ment. At first, the groove consists of a layer of flattened cells, 

 which are not to be distinguished from the other ectodermal cells ; 

 they soon, however, increase in number, and the groove itself is 

 thereby narrowed and flattened out, till it becomes converted into a 

 tube — the neural tube. About this time, the hinder portion of the 

 embryo also becomes altered ; the cells which were arranged in two 

 rows divide, and the hinder part gives rise to two median ridges, 

 which correspond to the germ-stripes of other Hirudinea. Segmenta- 

 tion commences early, and, after it has commenced, the embryo 

 undergoes a series of changes, in consequence of which the dorsal 

 surface ceases to be, and the ventral becomes convex. Prior to the 

 formation, which is somewhat late, of the coelom there is a cavity on 

 the dorsal surface between the ecto- and endoderm ; this is generally 

 anterior in position, and may be spoken of as the primary cceloin. 

 The oral invagination of the ectoderm gives rise to nothing but the 

 inner lining of the lips, all the other parts of the mouth and its 

 region being endodermal in origin. 



Development of the Central Nervous System of Annelids.* — It 

 is pointed out by J. W. Spengel, in an account of Kleinenberg's 

 recent researches on this subject, that we have here to do with the 

 development of new structures in the course of phylogenetic develop- 

 ment. The special larva studied appears to have been Lopadorhynchus, 

 which is of the Lovenian type. Simple as are most of the organs of 

 this form, the ectoderm presents considerable histological differentia- 

 tion ; a strong nerve lies in the groove formed on the ciliated girdle, 

 and this nerve is circular. On the upper hemisphere of the larva the 

 ectoderm is found to consist of large elements, immediately below 



* Biol. Centralbl., ii. (1882) pp. 231-6. 



