276 BASHFORD DEAN. 



stage in the writer^s material has he found any traces of either 

 anterior or posterior gill- slits to increase the normal number, 

 he is unable to accept Price^s suggestion, based upon the 

 number of spinal ganglia and the change in the position of the 

 gills, that the embryos of Bdellostoma have as many as thirty- 

 five pairs of gill-slits. 



Conclusions. 



The Myxinoid differs widely from all other Chordates in its 

 developmental type. Its sharply marked differences from the 

 mode of development of the lamprey emphasise the wide 

 divergence of these branches of the Marsipobranchian stem ; 

 and this alone forms a strong ground for belief in the antiquity 

 of the Cyclostomes, and for rejecting even a most remote 

 Teleostean ancestry. Developmentally the two branches cer- 

 tainly differ as widely as sharks and Ganoids. There can be 

 no doubt, furthermore, that the embryonic conditions of 

 Bdellostoma are not to be derived directly from those of the 

 lamprey. To what degree the converse is to be accepted must 

 remain for further study. We may now reasonably believe that 

 the ontogeny of a Myxinoid, when fully studied, will enable the 

 interrelationships of the Marsipobranchs to be broadly out- 

 lined : and it is possible that many valuable suggestions will 

 follow as to the general relations of these to Protochordates 

 on the one hand, and on the other to the ancestral Gnatho- 

 stome. 



Some of the more striking features in the development of 

 Bdellostoma, as above described, may finally be summarised. 



I. The neural tract is laid down, nearly in its entire length, 

 before the appearance of somites, and without any indication 

 of neuromeres. 



II. The neural tract, as in Petromyzon, acquires a lumen 

 by dissociation of cells (as shown by sections), proceeding 

 antero-posteriorly. 



III. The brain is distinctly a tubular structure, and differs 

 little in calibre from the spinal cord up to a relatively late 

 period in development, i. e. to the time of the appearance of 



