236 F. G. HBATHCOTE. 



development to give rise to fresh cells, which join the meso- 

 blast. This is exactly what happens in Julus. 



Metschnikeff has described the formation of the bands of 

 mesoblast and their division into somites, but his figures are 

 difficult to understand, as he has not drawn either the cell 

 outlines or the nuclei. 



The formation of the ventral flexure has been described by 

 Metschnikoff, and, as I have already mentioned, was first seen 

 by Newport. The flexure is, as I have before said, formed 

 between the sixth and seventh post-cephalic segments; that is, 

 it marks off from the rest of the body the long eighth segment 

 in which the tissues are very imperfectly diff'erentiated, and 

 from which the anal segment has yet to be cut ofi'. It is 

 from this imperfectly differentiated segment that the future 

 additional body segments are formed in the later stages of 

 development. 



The mesenteron of the adult animal is, as was pointed out 

 to me by the late Professor Balfour, marked with a series of 

 constrictions corresponding with the external segmentation of 

 the body, but no trace of such constrictions has as yet 

 appeared. 



The wide separation of the nerve-cords in the embryo has, 

 so far as I know, not been pointed out by any author. 



I propose to reserve for a future paper a more full descrip- 

 tion of the development of the nervous system, the circulatory 

 system, and the segmentation of the embryo, as well as the 

 account of the appendages and other points connected with the 

 further development of the embryo. 



Papees Referred to. 



1. Balbiani. — 'Generation des Vertebras," p. 258. 



2. BALfOTJB. — ' Comparative Embryology,' 1881. 



3. V. Beneden.— ' Pecondation de I'ceuf/ Liege, 1883. 



i. V. Caetjs. — " Entwicklung der Spinneneies," ' Zeitschr. fiir wiss. Zool.,' 



vol. ii. 

 5. Ctjviee.— ' Begne animal," 3rd ed., 1836, vol. ii, p. 330. 



