Normal Platcs of the Development of Lepidosiren paradoxa and Protopterus annectens. II 



Org^ans of the mesenchyme. 



As investigation of these organs is not yet nearly complete I-shall content myself with referring only 

 very briefly to some of the more important skeletal features which have been determined. 



Skeleton. 



It will be recalled that the notochord arises from the axial portion of the mass of medium sized 

 blastomeres lying dorsal to the archenteron. By stage 14 in Lepidosiren the mesoderm has become marked 

 off on each side by a split, the notochordal rudiment now forming a ridge like projection of the archenteric 

 roof. About stage 16 the chorda becomes split off from the thin layer of endoderm beneath it which per- 

 sists as the enteric roof. By about stage 23 the notochord has become cylindrical and a delicate primary 

 sheath is formed on its surface. As Agar (igo6 a) has shown, the front end of the notochord degenerates 

 leaving the sheath anteriorly filled with mesenchyme. Later on as the chordal cells increase in size as they 

 become vacuolated the notochord pushes its way forward again into the sheath, occupying the position of 

 the original front end which had degenerated and disappeared. About stage 32 the secondary sheath makes 

 its appearance. It rapidly increa.ses in thickness and about stage 36 (Lepidosiren) begins to be colonized 

 by Immigrant amoeboid cartilage cells from the arcualia, becoming eventually converted into a continuous 

 cylinder of cartilage. It is to be noted that the intracranial part of the secondary notochordal sheath be- 

 comes colonized by cartilage cells from the parachordals in precisely similar fashion. In the head region 

 the trabeculae appear first. The quadrate region of the mandibular arch is from the first continuous with 

 the trabeculae. A faint rudiment of the palatopterygoid outgrowth appears but soon disappears again (Agar). 

 The Suspension of the jaw apparatus is entirely by means of the upper end of the mandibular arch (proto- 

 stylic, Graham Kerr, 1907 b i) ; or autostylic — Gregory, 1904 — condition). The chondrocranium shows 

 progressive development towards the adult condition without signs of retrogression. 



Bone first makes its appearance about stage 32 — 34 in Lepidosiren in the form of thin sheaths investing 

 the base of the skull (parasphenoid) the pectoral girdle and hyoid arch, on the inner face of the lower 

 jaw (splenial) and along the side of the head from the quadrate forwards ("palatopterygoid" bone). By 

 stage 37 all of the individual bones of the adult skull have developed. 



1) I was unaware at the time of Gregory's paper of 1904, which renders my note in great part unnecessary. 



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