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



mesoderm belonging to a different region to that which their phylogenetic history 

 would lead us to expect, is shown by the great forward displacement of the peri- 

 cardium in embryos with much yolk, as in Lepidosiren and Proto'pterus. The cavity 

 of the pericardium here develops at the extreme front end of the head, under the 

 " stomodaeum," while the lateral plate in which it appears is still attached to the 

 somatic mesoderm. 



On the other hand, Lepidosiren and Protopterus do not furnish the evidence in 

 favour of a genetic distinction between a palingenetic and ccenogenetic somatic head 

 mesoderm which is shown by many Elasmobranchs, in which the occipital myotomes 

 migrate forwards during ontogeny, the anterior ones successively atrophying (Brads). 

 The occipital myotomes retain their original position (relative to the vagus) throughout 

 development. 



The essential facts about the segmentation of the head in Ceratodus, according to 

 Gregory, are the following : — The head mesoderm is at an early stage continuous with 

 the trunk mesoderm. At first unsegmented, it later becomes divided up, from behind 

 forwards, into four (pro-otic) segments. These correspond to the first four somites of 

 van Wijhe. The posterior segment is marked off by constrictions approximately over 

 the first and second gill pouches. These constrictions appear while the head mesoderm 

 is still connected with the trunk, so that the head and trunk segmentation are at this 

 stage continuous. All four somites have cavities. 



The fourth somite " soon loses its independence." 



The third is the hyoidean somite. It has a cavity, lined externally by epithelial 

 walls, its inner boundary being mesenchyme. It wanders forward internal to the second 

 somite, but the material did not allow of its fate being traced further. 



The second is the mandibular somite. It has a cavity like that of the third, but 

 continuous with the cavity in the mandibular arch. It sends forward a process on to 

 the dorsal edge of the eyeball, similar to that from which develops the obliquus 

 superior in Elasmobranchs. 



The first is the premandibular somite. The cavities on each side have epithelial 

 walls, and are connected by a solid strand across the middle line. It gives rise to out- 

 growths, from which presumably the oculomotorius muscles develop. 



As regards the fourth " somite," there seems no real reason for supposing it to have 

 the value of a single somite. It would better be described simply as a mass of somatic 

 mesoderm filling up the space between the hyoidean somite and the first occipital 

 myotome. 



Adding our knowledge of the constitution of the head mesoderm in the Dipnoi to 

 our previous knowledge of its condition in various Elasmobranchs, Amphibia, Reptiles, 

 and Birds, we find the result in favour of Gegenbaur's view, and may summarise the 

 whole, with most probability, as follows : — 



