410 



There are thus to be distinguished in the gull 6 anterior primary 

 rhombomeres, the first of which marks the place of origin of the 

 cerebellum and the succeeding five have the nerve relationships already 

 specified. The apparent posterior limit of the brain at an early phase 

 of development is therefore just behind the region from which the 

 abducent is to arise. It may be added that the fourth rhombomere 

 is from the beginning especially distinct. 



The six segments here defined have been recognised by early in- 

 vestigators, VON Baer, Remak, Durst, and by many recent workers. 

 But it has evidently been found difficult hitherto to determine the 

 relationship of the neuromeres to the nerves. Thus I conclude that 

 the six primary rhombomeres are homologous with the posterior 6 

 (6—11) of the neuromeres which have been described and figured by 

 Logy and by Hill in Acanthias, Salmo, and Gallus. Most of the 

 segments were observed by von Kupffer in many species, by Neal 

 in Acanthias, and by Waters in Gadus and Amblystoma. The last 

 of the six described by Sewertzoff in Ascalabotes corresponds with 

 my seventh rhombomere. The anterior four and apparently the seventh 

 and eighth were noted by Bradley in Sus, in which species the fifth 

 and sixth may not be clearly marked. 



But the metencephalon does not end at the sixth rhombomere, 

 nor at the eighth. A slight metameric segmentation of the hypoglossal 

 region of the metencephalon may also be made out, and these seg- 

 ments like the three last neuromeres in front of them alternate with 

 the myomeres. There are, at the stage when six rhombomeres are 

 established, seven occipito-spinal segments, or exactly seven and a half. 

 These are speedily reduced to five as the IX and X nerves develop. 

 The five surviving head myomeres are connected to the metencephalon 

 by five ventral roots, and opposite all but the first (= third) slender 

 rudimentary dorsal roots also make a transitory appearance (Fig. 4). 

 The myomeres and the ventral roots are reduced to four and sub- 

 sequently to three. The ventral roots of these last, together with the 

 first two spinal nerves form the hypoglossal. 



It is evident therefore that the neuromeres of the metencephalon 

 number 13, that myomeres reaching to the inter-rhombomeric position 

 6 — 7 occur at an early stage, and that these are reduced to five, four, 

 and finally to three (perhaps to two). 



Recent work has demonstrated that the number of so-called oc- 

 cipital somites is a very variable one. These head myomeres are from 

 3 to 4 in mammals, 4 or 5 or even 7 (Goronowitzsch) in birds and 

 reptiles, and amongst the Selachii they are still more numerous. 



