508 H. H. SWINNElxTON. 



from being slightly flexed (as shown by the dotted outline, 

 fio-. 57, c/i/jjhas now become bent back almost on itself {ch.). 

 When three days later the larva hatches out, all sign of 

 flexure has disappeared from the brain (fig. 58), and the 

 intercranial notochord itself has resumed approximately the 

 same shape as that which it exhibited within the egg (fig. 56) 

 on the sixth day. 



In a typical Elasmobranch's egg, that of Scyllium, the 

 embryo is at liberty to move about considerably, and there- 

 fore exists under conditions practically the same as those in 

 which the prematurely released or hatched stickleback exists. 

 Comparison of the structural peculiarities already referred to 

 in the Elasmobranch embryo must accordingly be, not with the 

 impi'isoned, but with the released stickleback. 



As to the cause of this flexure exhibited by the free 

 embryo (fig. 57), it is natural to expect that an elastic 

 body like the brain filled with fluid should, when liberated 

 from the pressure of the zona radiata, lose its flattened 

 form and assume a distended one ; but it is not so obvious 

 why such an alteration in external conditions should, on the 

 sixth day, produce a strongly marked flexure, and on the 

 ninth a slight straightening out (fig. 58). 



It is certain from the facts already put forward that the 

 cause for these unexpected effects is one which can be held 

 in abeyance by mechanical means, such as the resistance of 

 the zona, which disappears with advancing development. 

 The coincidence of the time of appearance of skeletal elements 

 with the time of great flexure suggests that these may be one 

 factor in that cause. Another factor is perhaps to be found 

 in the great difference in the degree of development between 

 the dorsal and ventral portions of the brain ; for whilst the 

 latter is very massive, the former is at present little more 

 than an epithelium (figs. 54, 55, h.h.). A comparison of 

 figs. 57 and 58 will show that on the sixth day the skeletal 

 elements {tr. and ch.), as compared with the brain, are 

 relatively much shorter than on the ninth day. The fact 

 that in the released embryo (fig. 57) the optic nerve (11 j. 



