24 SUMMAEY OF CUKRENT RESEAECHES RELATING TO 



pectoral or pelvic girdle, and runs parallel to the long axis of the 

 body. The free end of the bar begins to undergo segmentation into 

 rays, and much of this is effected " before the tissue of which the 

 plates are formed is sufficiently differentiated to be called cartilage by 

 an histologist." 



We have then a longitudinal bar along the base of the fin, which 

 gives off perpendicularly a series of rays which pass into the fin. It 

 is pointed out that, from its position this basal piece can never have 

 been a median axial bar with rays on both sides. The resemblance 

 to the arrangement of the impaired fins is consequently very striking, 

 and support is given to the author's original doctrine of a once 

 continuous lateral fin. 



Development of the Sturgeon.* — In continuation of his previous 

 paper. Professor W. Salensky points out that in this fish it is very 

 difficult to fix the limits between the period of the formation of the 

 embryonic layers and that in which there appear the earliest rudi- 

 ments of the organs. Here we find that the envelopment of the 

 inferior by the superior portion, and the further differentiation of the 

 embryonic layei's is contemporaneous with the appearance of some of 

 the organs in the mesoblast. Dealing with the modifications under- 

 gone by the egg up to the point at which the medullary groove 

 becomes closed, the author states that organs begin to appear at the 

 termination of the first day of development. On the second day a 

 groove 0"7"' in length appears in the middle of the embryonic area. 

 The posterior extremity of this groove corresponds exactly to the 

 blastopore. In the next stage the anterior end of this primitive 

 groove dilates to form the rhomboidal rudiment of the brain. The 

 hinder part of the groove opens directly into the primitive digestive 

 cavity by means of the blastopore, and it is only near the end of the 

 period of development that the union between the digestive and 

 medullary cavities ceases to exist. Meantime, the lateral parts of the 

 embryonic area have been undergoing important changes. On either 

 side there appears a white band which behind diverges slightly 

 from its fellow. These are the first indications of the Wolffian ducj;s ; 

 and the parts internal to them become modified to form the vertebral 

 plates, and those external to them the lateral plates. 



Previous, however, to the appearance of the groove on the surface 

 of the embryonic area, important changes have been taking place 

 within. There has appeared an axial thickening, formed from the 

 ectoderm and mesoderm, which has an intimate connection with the 

 formation of the notochord and of the central nervous system. These 

 changes are described in detail. The mesoderm becomes divided into 

 a median and lateral portions ; the first constitutes the notochord, 

 while the side pieces give rise to various organs. After the appear- 

 ance of the medullary groove we may distinguish a central portion in 

 which the groove is placed, and lateral parts which are distinguished 

 by having over them the enveloping lamella. The bases of the cells 

 which form the floor of the groove are strongly pigmented, and this 



* Arch, de Biol., ii. (1881) pp. 279-341 (4 pis.). 



