198 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 



section, that the greater part of the cells proliferated into the cavity of 

 the myotome, the cells of which are at this stage already converted into 

 elongated muscle cells, arise from its median wall. While the outer 

 Avail still maintains its primitive epithelial character, the inner wall has 

 become many cells in thickness and some of these cells appear in the 

 act of migrating into the now greatly diminished lumen of the cavitv. 

 Later, however, the cells of the outer wall also are converted into muscle 

 cells, and thus both walls of the cavity participate in the formation of the 

 muse, rectus posterior. We have therefore in the 3d somite of van Wijhe 

 a pi-e-otic segment of the dorsal mesoderm, which becomes differentiated 

 into myotome and sclerotome, and whose musculature is derived in greater 

 part from its median wall. Furthermore, as is well known, its musculature 

 is innervated by a nerve (abducens) which all the later morphologists, with, 

 so far as I know, one exception (Kupfler, '94, '95), regard as a ventral 

 nerve comparable with spinal ventral nei-ves. Finding this to be the case 

 with at least one pre-otic mesoderm segment, we are in a better position 

 than we otherwise should be to understand the more modified, or at least 

 more divergent, conditions presented by the remaining pre-otic segments, 

 viz. the anterior, the 1st, the 2d, and the 4th. That in these segments 

 marked peculiarities appear is certain. In the 4th somite we have a 

 segment of the dorsal mesodei'm divided by constrictions from the 3d 

 and 5th somites at a time when it presents essentially the same evidences 

 of differentiation into myotome and sclerotome "which appear in the 3d 

 and 5th somites. That no muscle cells are formed in its inner wall, and 

 that it soon breaks up into loose mesenchyma, are phenomena which are 

 to be expected in a somite destined to become rudimentary. That it is 

 more rudimentary than the 5th somite is due to the development of the 

 otic capsule, under which it lies. The 5th somite — in whose inner w^all 

 elongated cells appear, without however developing into muscle fibres 

 (as stated by Sedgwick, '92) — thus forms a natural transition to the con- 

 ditions presented by the 4th. If the 3d and 5th are to rank as somites, 

 it is in my opinion impossible to deny that the 4th, which lies between 

 them, is serially homologous with them, even though it should lack some 

 of the characteristics of a typical trunk somite. 



Passing forward in the embryo to the 2d (mandibular) somite, it 

 seems to me indisputable that this is the anterior continuation of the 

 dorsnl mesoderm. In very earh' stages it grows ventrally to form the 

 mesoderm (mesothelium) of the mandibular arch, a process which, accord- 

 ing to Kupffer ('88), occurs in Petromyzon also. However, only the 

 dorsal part of the " mandibular cavity," which later becomes separated 



