ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 445 



ditions in Selachians, or even in Teleosts, although, unlike what occurs 

 in the latter, specialisation has not proceeded so far as to cause the 

 development of a definite germ-ring. In no developmental characters, 

 purhaps, do the tw>> types of Cyclostomes stand more widely apart than 

 in the processes connected with g istrulation. 



The origin of the lumen in the central nervous system of the Myxi- 

 nuid by infolding medullary folds is a feature of the greatest signifi- 

 cance. For evidently, if this character has here beon retained, in spite 

 of the unfavourable conditions of yolk and of restricting membranes, it 

 is clearly of great palingenetic value. The great length of the brain in 

 the embryos of Bdellostoma is also important. As to auditory and optic 

 organs, there is no tangible evidence of degeneration ; the Myxinoid 

 gives little ground for the belief that the pineal eye was a primitive 

 character of the Crauiota ; nor is there any obvious evidence that an 

 embryonic series of end-organs is present comparable to a distinct 

 lateral line. 



The head-structures in Bdellostoma are strikingly different from those 

 in the lamprey. The noteworthy flattening of the heal at early stages 

 has borne with it the modification of the stoniodaeal region, and, for the 

 present at least, it is impossible to say to what point in the mouth-region 

 of a young hag the primitive ectoderm extends. The actual mouth is 

 clearly a neomorph, and the " velum " is in a far more superficial 

 position than the velum of the lamprey. As to the hypophysis, the 

 evidence is clearly favourable to the view of von Kuppfer as to its 

 palaeostomal nature. The fact thtit its posterior part is distinctly paired 

 recalls the early work of Dohrn in homologising the structure with gill- 

 slits. The embryonic nasal sacs arise from the early Nasenrachengang as 

 paired structures. 



The resemblance of the embryonic mouth to that of a Gnathostome 

 is very striking. Every step of transition occurs between the narrow 

 transversely directed mouth-cavity of the embryo and the constricted and 

 barbel-filled oral lobe of the adult. 



No structures directly comparable to visceral arches occur at any 

 stage. The various skeletal elements which occur in late embryos in 

 the hinder region of the head have probably arisen as neomorphs in 

 especial relation to the muscles of the barbels and tongue. These 

 elements certainly do not appear until after the gill-slits have passed 

 entirely out of their neighbourhood. Nor is there any hint of paired 

 fins. As regards gill-pouches, there is a closer correspondence between 

 Myxinoid and lamprey than one would at first expect, but in general 

 the gill-pouches of hags have retained the simpler conditions. The 

 excretory system of Myxinoids is generally admitted to be more primi- 

 tive thau that of the lamprey, and this conclusion is now greatly 

 strengthened by the discovery that a series of segmental tubules, in all 

 probability pronephric, is found to occur throughout practically the entire 

 body-length of the embryo. 



It seems fair to conclude that the Myxinoids aiv, not to be looked 

 upon as forms which have become greatly degenerate. As to general 

 morphological conclusions, the author lays emphasis on the fact that in 

 Myxinoids branchiomery does not correspond with myomery. The 

 suggestion is also made that the vertebrate brain is not a collection of 



Aug. 15th, 1900 2 H 



