[49] EMBEYOGRAPHY OF OSSEOUS FISHES. 503 



germinal matter concerned in building up new structures in the way in 

 which we see that the tail is evolved we do not know. And yet it is 

 very hard to see how it is possible for new material to pass through and 

 around the cells and cellular structures already built up, to reach the 

 extremity of the tail in order to add to its length and bulk. 



11. — The development of the brain. 



The brain or encephalon of the embryo cod, on the fourteenth day, 

 has somewhat the form of a vertical flat rhomboidal sack, its interior 

 bluntly pointed extremity being the rudiment of the cerebrum, and its 

 hinder part is continued into the anterior end of the neurula or neural 

 tube, as the embryonic spinal cord may now be called. On the tenth 

 day it already begins to become thicker behind the posterior borders of 

 the optic vesicles. By the eleventh day a distinct constriction of the 

 cephalic end of the neurula behind the eyes divides the latter into an 

 anterior and i^osterior portion, as shown in Fig. 27, but it can hardly 

 as yet be said that a cerebral vesicle has developed, for there is now 

 present only a vertical cleft in the center of the cerebral end ev of the 

 neurula. Tlie walls of the brain of Teleostean embryos of this stage, 

 unlike those of other types, are now very thick. They consist in fact 

 of two thick flat plates of cells placed vertically between the eyes. The 

 first constriction tf of Fig. 27 marks the boundary between the mid- 

 brain and the cerebellum. At a later period a constriction appears a 

 little way behind this one which marks off the cerebellum and medulla 

 oblongata from each other. » This occurs about the twelfth day, when 

 the fore part of the brain rudiment also acquires another constriction 

 which separates the mid-brain from the cerebrum or fore-brain as shown 

 in Figs. 29 and 30. By the fourteenth day the cerebral regions have 

 been developed and the first, second, third, and fourth vesicles or cerebral 

 cavities are present, but they still retain the laterally compressed form 

 characteristic of the early stages of Teleostean brain-development. 



The so-called ventral bend or flexure of the encephalon in the embryos 

 of other types is almost null in the Teleostean embryo. In fact, I much 

 doubt if it can be shown that any flexure occurs, as the development 

 of the brain of the osseous fish can be accounted for on another princi- 

 ple. With the great development of the ventral keel of the neurula at 

 the head end of the embryo the vertical depth of the encephalon is 

 almost as great as when the cerebral vesicles are developed. If an in- 

 vagination upwards and forwards of the floor of the brain now takes 

 place, so as to develop the infundibulum in front of it, the construc- 

 tion of the brain of the late stages is attained. My reason for holding 

 that the infundibulum is developed in this way, is the fact that no per- 

 ceptible downward flexure of the encephalon ever occurs prior to the 

 development of that portion of the brain. Moreover, the infundibulum 

 is developed long before the head becomes free from the yelk-sack, and 

 therefore before a downward flexure of the brain is possible to any 



