Johnston, Morphology of the Head. 185 



a. A continuous series of mesodermic somites appears in 

 early embryos of lower vertebrates. The earliest somite to 

 appear lies in the neck region and others are formed progress- 

 ively forward and backward from this point. The order of their 

 appearance and the continuity of the series justifies the conclu- 

 sion that the somites of head and trunk belong to a single series. 



b. That somite of this series which van VVijhe numbered 

 somite 2 always lies dorsal to and is continuous with the man- 

 dibular arch. The relation of the succeeding somites to the 

 branchial arches is more or less complicated. See Sec. 3 below. 



c. In front of this lie two somites, the praemandibular, 

 or somite i of van Wijhe, and the anterior head cavity of Platt 

 (119, 124, 106, 57). 



d. Somites i, 2 and 3 give rise to the eye muscles in all 

 vertebrates. 



e. The anterior head cavity of Platt never persists. 



The question whether somite i is composed wholly of dor- 

 sal mesoderm and whether the mesoderm anterior to it is dorsal 

 or lateral may be discussed here. It is a question which can 

 scarcely be answered by direct observation. The exact condi- 

 tion of the mesoderm in this region in cyclostomes is uncer- 

 tain. KuPFFER has described and figured for Ammocoetes 

 (78) well developed vestiges of branchial and aortic arches sur- 

 rounding the hypophysial canal, the relationships being the 

 same as in the pharyngeal region ; but this has not been con- 

 firmed. His argument from the position of these arches that 

 the (whole of the) praemandibular mesoderm constitutes a lat- 

 eral or branchiomeric segment is negatived by the fact (stated 

 by himself also) that the dorsal part of this mesoderm forms an 

 eye muscle which is innervated by a true ventral nerve in all 

 vertebrates. In selachians also, wiiere the mesoderm is better 

 preserved, the embryonic history fails to give any positive evi- 

 dence of the existence of branchial arch mesoderm in front of 

 the mandibular arch. 



One important consideration is to noticed here, however,, 

 which has a bearing on the whole problem of segmentation. 

 It is that in the anterior part of the head in all lower verte- 



