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



thelium), and the connection left between the first and second outgrowths corresponds 

 to the hyoidean cavity (also a solid mesothelium). Behind this there is another section 

 of mesothelium cut off between the second and third outgrowths, and behind this the 

 lateral plate and dorsal mesoderm are connected by a continuous sheet of mesothelium. 



There is no trace of segmentation of the dorsal or somatic mesoderm of the head at 

 this or any other stage, and there is no reason to regard the portions of mesothelium 

 cut off between the gill outgrowths as having any segmental value. It may be specially 

 noted that there is no indication of a premandibular cavity. As will be seen later on, 

 however, in discussing the development of the eye and mandibular muscles, there is 

 some reason to believe that regions corresponding with the somatic portions of the first 

 three head somites of Balfour and van Wijhe (premandibular, mandibular, and 

 hyoidean) are differentiated from one another. 



In the trunk, at this stage, the lateral plate is unsplit. In the head, the somato- 



Fig. 1. — Lepidosiren, stage 20, transverse section between the first and second gill outgrowths. 

 br.r., branchial ridge; int.m., intermediate mesoderm; l.p., lateral plate; mes., 

 mesenchyme ; pcdm. , cavity in lateral plate representing first rudiment of pericardial 

 cavity ; som., somatic or myotomic head mesoderm. 



pleural layer of mesoderm has separated from the splanchnopleural in the region ventra] 

 to the first two gill outgrowths, forming the first, paired, rudiments of the pericardium. 

 The lateral plates of the two sides are still wide apart throughout their whole length, 

 owing to the fact that the pharynx and " stomodseum " have not yet become folded off 

 from the yolk sac, over the surface of which the lateral plates are spread out. 



At stage 23 + the lateral plate in the trunk has separated from the myotomes. 

 As before, it stretches up without a break from the trunk into the head, but the series 

 of trunk myotomes is now sharply marked off from the somatic head mesoderm, the 

 latter having almost completely broken up into loose mesenchyme. The front end of 

 the trunk series is a short distance behind the auditory vesicle, — the actual distance 

 varies considerably, but is usually rather more than the length of one myotome. 



At the last stage we saw that the three gill outgrowths first formed cut off between 

 them portions of the mesothelium connecting the lateral plate and somatic mesoderm. 

 When the posterior gill outgrowths are formed, they do not, however, cut off similar 

 portions of mesothelium, this having now entirely resolved itself into mesenchyme, 



