ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 25 



pigment is derived from the ectodermal cells which become confoimded 

 with the cells of the medullary plates. 



The further development of the nervous system consists in the 

 progressive development of the lateral pieces which correspond to the 

 medullary plates of other Vertebrates. At about this stage the parts of 

 the mesoderm give rise to the excretory organs. 



After the medullary groove becomes closed it is possible to dis- 

 tinguish a cephalic from a trunk region, and the boundary between 

 the two corresponds to the anterior ends of the Wolffian bodies. 

 Owing to the transparency of the embryonic area it is possible to see 

 that the trunk grows by a gradual increase in the number of the 

 primitive segments. Having before had five somites, we see these 

 last increase as changes go on in the form and position of the blasto- 

 pore. While the anterior segments retain their perpendicular position, 

 the posterior become inclined to the longitudinal axis, to return later 

 on to their primitive position. While elongation is proceeding, the 

 trunk becomes thicker, the dorsal region increases in size, and there 

 is exhibited a slight inclination to the right, the appearance of which 

 causes a certain asymmetry in sections taken at this period. 



Soon the blastopore closes, and its position is marked by an accu- 

 mulation of pigment. The rudiment of the tail becomes visible by 

 the formation of a tubercle ; the cephalic extremity possesses two 

 vesicles, and the mesoderm is still thin anteriorly. Where the 

 cephalic plate enlarges, a central and a peripheral part may be distin- 

 guished, the branchial clefts begin to appear, and a facial process is 

 developed in front of the head. The heart does not commence to con- 

 tract till the end of the period of embryonic development, and its 

 contractions are at first very slow. Simultaneously with this the 

 veins and their ramifications appear. 



The author next proceeds to a study of the development of the 

 internal organs with which he considers the modifications of the 

 embryonic layers. He points out that the ectoderm consists of two 

 layers, of which the superior is, at first, strongly pigmented ; through- 

 out its development its cells nearly all retain their original flattened 

 character ; the lower layer is that which contributes most largely to 

 the formation of the sensory organs in which, except in the case of the 

 olfactory fossae, the outer layer takes no part. After describing the 

 details of the development of the central nervous system. Professor 

 Salensky raises the question of the homology of this region with 

 the nervous system of Vermes and Arthropoda. He points out that 

 (1) the central nervous system of all Vertebrates is formed from 

 two thickenings of the ectoderm, set parallel to the long axis of the 

 body : that of all Articulates has a similar origin. (2) In some cases, 

 e. g. Echiurus, the " Articulates " present a median groove comparable 

 to that of Vertebrates. (3) The formation of the medullary groove 

 commences, in the case of both phyla, posteriorly, and is continued 

 forwards. On the other hand the Vertebrata have the central nervous 

 system dorsal in position, and the medullary groove becomes closed. 

 As to the first of these, he points out that the position of the mouth 

 is the determining character, in conjunction with that of the loco- 



