Meek, The Segments of the Head. 



213 



of the hind brain, have attracted attention. Our knowledge 

 with regard to these primitive folds or segments of the neural 

 plate and canal has been greatly advanced by the contributions 

 made by Dursy, Goette, Dohrn, Mihalkovics, and 

 v. Kupffer, and by the American authors, McClure, Locy, 

 N e a 1 and Hill. In the case of the Lesser Black- 

 backed Gull Larus fuscus L., I have not been able to see the 

 complete segmentation of the neural plate as it has been 

 figured by the American writers, for in the gull the neural folds 

 are approximated in the region of the prosencephalon and mesen- 

 cephalon before the greater portion of the metencephalon has 

 become visible, and before the more conspicuous of the head 

 somites have been formed. As is piain from the accompanying 

 figure (fig. 1) the neural folds rise and meet in an antero-posterior 

 direction. The margins of the folds show the primitive segments, 

 and the segmentation ex- 

 tends to the outer walls 

 of the folds and to the 

 floor of the neural plate. 

 Thus when fusion takes 

 place the medullary canal 

 is resolved into a series of 

 vesicles separated by trans- 

 verse constrictions of the 

 lateral walls. These are 

 the neuromeres, and when 

 the somites are formed 

 they are seen to alternate 

 with the latter. 



From the work which 

 has been referred to above 

 it has become piain that 

 during Craniate ontogeny 

 the three anterior seg- 

 ments are concerned in 

 the formation of the pro- 

 sencephalon, and of these 

 prosomeres the first is 

 olfactory and the second 



is optic; that the mesencephalon is formed from two mesomeres 

 which yield the oculomotor and the trochlear ; and that the metence- 

 phalon presents six rhombomeres anteriorly. The second of these 

 is connected with the trigeminal, and the fourth with the acustico- 

 facialis-I have been able to show with reference to a number of 

 Craniates that the fifth and sixth yield the roots of the abducent, 

 and also that the seventh and eighth are related to the glosso- 

 pharyngeal and vagus respectively. 



Fig. 1. Dorsal view of embryo Gull, beginning 

 of second day, with reconstruction of somites. 



