( 611 ) 



XXIII. — The Development of the Anterior Mesoderm, and Paired Fins with their 

 Nerves, in Lepidosiren and Protopterus. By W. E. Agar, B.A., Junior 

 Assistant in Zoology at Glasgow University. Communicated by Professor J. 

 Graham Kerr. (With a Plate.) 



(MS. received December 7, 1906. Read February 4, 1907. Issued separately May 17, 1907.) 



CONTENTS. 



II. 



The Head Mesoderm, 611 



Ia. Segmentation and Musculature of the 



Head, ....... 613 



Ib. Lateral Plate Mesoderm in Head and 



Neck, and Constrictor pharyngis, . . 619 

 The Occipital and Anterior Trunk Myotomes and 



Nerves, 623 



11a. Cervical (Hypoglossal) and Brachial 



Plexuses and Occipital Nerves, . . 623 



III. 

 IV. 



IIb. Embryonic Condition of the Occipital and 



Anterior Trunk Nerves and Myotomes, 624 

 lie. Derivatives of the Anterior Trunk 



Myotomes, 625 



Development of the Coraco-hyoid Muscle, 627 



Development of the Pectoral Fin, . . 627 



Development of the Pelvic Fin, .... 631 



Summary, 635 



List of papers referred to, 



637 



I. The Head Mesoderm.* 



In a Lepidosiren embryo of stage 20 (Graham Kerr) the mesoderm stretches 

 continuously through the head and trunk regions. In the trunk the dorsal mesoderm is 

 segmented into myotomes, but the segmentation stops short some distance behind the 

 auditory vesicle, leaving unsegmented mesoderm in front of this. In both head and 

 trunk the lateral plate mesoderm is still connected with the dorsal, myotomic or somatic 

 portion. 



In the head, the mesoderm reaches up on each side to the front end of the optic 

 vesicle. It consists of (1) compact mesoderm, composed of a dorsal, somatic portion, 

 continuous ventro-laterally with the lateral plate, and (2) a loose mesenchyme between 

 the somatic mesoderm and the ectoderm (text figure 1). This mesenchyme is being 

 formed by the breaking up of the compact mesoderm, as can be seen by following up 

 successive stages, when it becomes apparent that the former is increasing at the expense 

 of the latter. 



At this stage there are three solid gill outgrowths from the solid pharynx on each 

 side. They are connected by a ridge, — branchial ridge (text figure l). These gill 

 outgrowths break through the intermediate mesoderm connecting the somatic and 

 lateral plate mesoderm (text figure 1), so that the latter, being entirely ventral to the 

 outgrowths, is uninterrupted. The intermediate mesoderm connecting the somatic 

 mesoderm and lateral plate in front of the first outgrowth corresponds with the man- 

 dibular cavity of Balfour and van Wijhe (though it is, as a matter of fact, a solid meso- 



* The material for this investigation was that in possession of Professor J. Graham Kerr, who kindly allowed 

 me to make use of it for the purpose, and in whose laboratory the work has been done. 



This thesis formed part of one of the essays successful in gaining the Walsingham Medal of Cambridge University 

 in 1906. 



TRANS. ROY. SOC. EDIN., VOL. XLV. PART III. (NO. 23). 87 



