TKANSACTIONS Ol' THE SKCTIOXS. 141 



scvciitli opposite the fourteentli and fifteenth septa ; and this is one reason for 

 douhtiug that even these superlicial branchial cartihiges, though attached to the 

 vertebral column, are to be regarded as libs. 



" It may be noticed as a wholesome symptom in anatomical speculation, that the 

 new theory which has led to these remarks is founded on arguments drawn alto- 

 gether from comparison of different species, and not from embryology, a very 

 remarkable circumstance as coming from one who so lately as last autumn reite- 

 rated in this Section his slowness to believe in reasonings founded on adult forms, 

 and even on " later development." The wisest know so little, that humanity must 

 be consent to gather information from every possible soiu-ce, and leave no set of 

 ascertained facts out of view in attempting to arrive at generalizations. If we had 

 before us all the adult anatomy of every species that ever lived on the earth, we 

 should only then have the record completed from which to frame a full system of 

 morphology ; and as matters stand we must translate embryological phenomena 

 with the aid of the series of adult forms, as well as translate the teachings of the 

 adult series with the aid of embryology. • 



Falling back on my proposition, that the segments of the vertebrate body are 

 nowhere complete, and that segmentation at one depth may exist to a gi-eater ex- 

 tent than at another, I may mention certain embryological phenomena in the brain, 

 which have received too little attention, and which to some extent warrant belief 

 in a larger number of segments in the head than is usually admitted ; although I 

 do not see that they are necessarily at variance with that theory of seven segments 

 in every ossified sl-iull which I indicated in 18C>2. In the chick, in the middle of 

 the seciand day of hatching, already is the third cerebral vesicle divided into a 

 series of five parts, separated by slight constrictions, the first part larger-than those 

 which succeed, and the last part narrowing to the spinal cord. The auditory 

 vesicle lies opposite the constriction between the fourth and fifth parts. At the 

 end of the second day and dm-ing the third, these divisions assume dimensions 

 which give them a general appearance exceedingly similar in profile to the proto- 

 vertebrie of the neck. In the following day they exhibit a more complex appear- 

 ance, and after that the first compartment alone remains distinct as cerebellum, 

 while the divisions between the others disappear in the thickening of the cerebral 

 walls. In their first two stages, Mr. Huxley, whom I have already referred to so 

 often, has figured these crenations, but he has not, so far as I know, described 

 them. 



I may also direct attention to another embryological point, to which I referred 

 last year at Belfast as a probability. I speak now from observation. That which 

 is termed tlie first cerebral vesicle in the early part of the second day of hatching 

 of the chick, is an undifferentiated region of the brain from whicTi a number of 

 parts emerge successively from behind forwards. As early as the thirty-sixth hoiu* 

 the optic nerves can be traced, separated from the rest of the vesicle by distinct 

 elevations of the floor of the brain, reaching inwards to the constriction between 

 the first and second vesicles : and as early as this date the first trace of bifidity of 

 the brain in front may be discerned — that bifidity which, to my thinking, is only 

 one of several instances of longitudinal fission in the fore part of the head, the 

 trabeculse presenting another instance of the same thing, and the cleft between the 

 maxillary lobe and the part of the head above it a third ; while in the muscular 

 svstem such longitudinal cleavage or fission is common even in the trunk. In a 

 chick of the third or fourth day, when rendered very transparent, the optic nerves 

 can be seen extending from beneath the front of the optic lobes ; while in front of 

 the optic lobes there are placed in series from behind forwards a posterior division 

 of the first vesicle, an anterior division, the cerebral hemispheres, and the olfactory 

 lobes. Thus there is a large supply of material presented in the brain for the study 

 of segmentation ; the difficulty to be overcome by future inquiry and careful col- 

 lation of all available facts is to determine the value of the parts placed one in 

 front of another. 



Perhaps I have occupied time too long with matters involving a large amount 

 of technical detail ; but I trust that I may have, in some measure, illustrated that 

 both in aim and in accomplished work Anatomy is no mere collection of discon- 

 nected facts, no mere handmaid of the physician and surgeon, nor even of Phy- 



