614 MR W. E. AGAR ON DEVELOPMENT OF ANTERIOR MESODERM, AND 



the whole mesenchyme from which they arise becomes merged into a continuous mass 

 before the individual muscles are differentiated. The posterior extension of the eye 

 muscle rudiment disappears by shortening up from behind, about stage 32. 



This greatly modified mode of development of the eye muscles may probably be 

 partly referred to the reduced condition of these organs in the adult, but to a greater 

 extent to the large amount of yolk in the tissues. 



Fig. 2. 



Fig. 3. 



Fig. 4. 



Figs. 2, 3, 4. — Three sections from the same individual of Protopterus, stage 30, separated hy spaces of 50ju. 

 Fig. 2 is the most anterior, e.m.r. , eye muscle rudiment ; e.m.r.p., backward extension of this rudi- 

 ment : lit., heart ; o.v., optic vesicle; ph., pharynx ; t.m.r., rudiment of temporal muscle. 



As regards the metotic head " somites " of van Wijhe (5-9 van Wijhe, 4-9 many 

 authors), two views are held : — 



1. That they belong to the head proper, their splanchnic portions giving rise to the 

 muscles of the branchial arches, the occipital nerves representing their somatic motor 

 nerves, and their splanchnic, motor and sensory nerves being "collected" into the 

 vagus (van Wijhe, Miss Platt (table, p. 454), Koltzoff, Johnston, etc.). 



2. That they properly belong to a part of the trunk which has only lately become 

 included in the hinder part of the head, and the mesoderm included in the visceral 

 arches, though actually ventral to them (the metotic myotomes), belongs morphologi- 

 cally to the region in front of them. There is on this view a sharp break between the 

 mesoderm of the head proper and these metotic or occipital myotomes, marking two 

 genetically distinct parts of the head (spinal and pre-spinal, Froriep ; ccenogenetic 

 and palingenetic, GeCxENBAur ; neo- and paleo-cranium, Furbringer). The posterior of 

 these two regions (occipital myotomes) is regarded as having now come secondarily to 



