FORMATION OF THE MUSCLES OF THE HEAD. 161 



posterior) direction (fig. 191, i.m.), and it may presently be observed that they arc 

 becoming developed into longitudinal groups . or segments of muscle-fibres which 

 stretch between the original intervals between the protovertebrse, The destination 

 of the outer layer of the muscle-plate has not been traced with certainty. Balfour 



Fig. 191. HORIZONTAL LONGITUDINAL 



SECTION OF THREE PROTOVERTE- 

 BRJ3 IN A SNAKE EMBRYO. 



(v. Elmer.) 



cp, cutaneous epiblast ; c. m, exter- 

 nal la.yer of muscle-plate ; /, its mar- 

 gins folded round into i.m, internal 

 layer of muscle-plate composed of 

 flattened cells which are becoming 

 elongated into muscular fibres ; n.c, 

 neural canal, in outline only ; n'.c', 

 neural epiblast forming its walls. 

 Between these and the muscle-plate ' 



is a continuous mesoblastie tissue 



which has been derived from the inner parts of the protovertebrse, partly interrupted by the ganglion 

 rudiments, yl. The original intervals between the protovertebrse here are still indicated by vessels, v. 

 i.cl, cleft in the deeper proto vertebral tissue (according to Ebner this is the remains of the original 

 protovertebral cavity). 



described it as also eventually becoming transformed into muscle-fibres (in elasmo- 

 branchs), but others have failed to confirm this opinion, and it is by some believed 

 that it may assist in the formation of the cutis vera. 



Although the muscle-plates are originally mainly concerned with the formation 

 of the muscles which move the central skeletal axis, it is probable that all the 

 skeletal muscles both of the trunk and limbs are eventually derived from them (see 

 below, p. 163). 



Formation of the muscles of the head and evidences of head segmentation. 

 Although, perhaps no part of the cranium actually represents a vertebra, there is nevertheless 

 abundant evidence of an original segmentation of the head corresponding 1 with the mesoblastie 

 somites of the trunk. Such segmentation is shown by the existence of the visceral arches, 

 which in the typical and least modified vertebrates (e.g., elasmobranchs) are at least nine 

 in number, by the successive separation of the part of the body cavity which extends into 

 the head into separate portions, or head cavities, one corresponding to each visceral arch, the 

 parietes of which develop into muscles, and which therefore correspond with the muscle- 

 plates of the protovertebrae of the trunk (this is in fact the typical mode of formation of 

 mesoblastie somites, v. page 26), and lastly by the mode of development of the cranial 

 nerves and their relations to the visceral and branchial arches, which correspond in a general 

 way with the relations of the ventral branches of the spinal nerves to the ribs. 



The formation and destination of the head cavities have been investigated of late years 

 (chiefly in elasmobranchs, but also in reptiles and birds) by Balfour, Milnes Marshall, and 

 van Wijhe, and the result of these investigations tends to show that, in all. nine portions of 

 the original head cavity become separated off on either side, their formation proceeding from 

 behind forwards. Each somite cavity becomes subsequently divided into a dorsal part corre- 

 sponding with the protovertebra3 of the trunk and a ventral part, corresponding with the 

 pleuroperitoneal cavity, and lying in the middle of the corresponding visceral arch ( ! ). Both 

 parts give rise by differentiation of their parietes to muscles ; the visceral arch portions to 

 the muscles of the jaw and hyoidean and branchial apparatus ; the dorsal portions, some (first, 

 second, and third) to the muscles which move the eyes, some (seventh, eighth, and ninth) to 

 the muscles which connect the head with the shoulder girdle, whilst some, viz., the fourth, 

 fifth, and sixth, are said to disappear. The first head cavity forms the eye muscles, which arc 

 -supplied by the third nerve ; the second, the muscle supplied by the fourth (superior oblique) ; 

 and the third, the muscle supplied by the sixth (external rectus). In higher vertebrates, the 

 formation of the head cavities and their subsequent destination have not been as yet clearly 

 followed out, although indications of their existence are not wanting. 



1 According to v. Wijhe the intermediate part of the typical somite cavity represents a segmental 

 (ariniferous) organ, but this intermediate part is not seen in the head, although it begins to appear in 

 the immediately succeeding somites. 



VOL. I. M 



