132 



BIOLOGICAL LECTURES. 



cranial nerves have begun to develop and show definite relations 

 with some of the neuromeres of the hind-brain. For example, 

 as is best shown in Fig. 7, the fifth nerve is connected with the 

 first and second neuromeres of the hind-brain, that is, with the 

 segments Nos. 6 and 7. The eighth neuromere has no nerve 



connection ; the seventh and eighth 

 nerves are connected with the ninth 

 and tenth neuromeres ; the ninth nerve 

 with the eleventh neuromere, and the 

 front root of the vagus is connected 

 with the twelfth segment. 



The evidence of a primitive head 

 segmentation, which is so well preserved 

 in these animals, is by no means ex- 

 ceptional, as may be determined by 

 examining the embryos of other animals. 

 They are present at least in corre- 

 spondingly young stages of birds and 

 amphibia, and this considerable range 

 indicates they are a fundamental char- 

 acteristic. 



In the very young chick, I have 

 repeatedly examined them in living 

 specimens. They are to be faintly 

 seen as early as the twelfth to the 

 fifteenth hour of incubation ; and from 

 that time onwards they are ever present 

 till they are obliterated by transforma- 

 tions in the brain. In the earlier 

 stages in which I have observed them 

 there are only three pairs, and they 

 apparently increase in number by backward growth. Fig. 8 

 shows the appearance of these segments in a chick embryo 

 while the neural groove is open. The segments extend from 

 the anterior limit of the head as far back as the neural folds are 

 established; there are here four protovertebr^, but lying in 

 front of them and entirely distinct from them are eleven pairs 

 of the primitive neural segments. 



Fig. 8. — Embryo of chick, with 

 open neural groove and three 

 well-marked mesoblastic so- 

 mites. The neural folds are 

 segmented throughout their 

 extent. 



