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



between pronephros, ventral process M3, and ectoderm. The loose mesenchymatous 

 mass then spreads out below the pronephros, occupying its proper position ventral to 

 J\l2, but separated from it by the pronephros. It is in this condition in stage 30 

 (Plate fig. 4 ; shown also in Protopterus in text figure 7 — v.p. 2). 



The ventral process of M3, which does not separate from its myotome like that of 

 M2, is shown in Plate fig. 5, and under a higher magnification in fig. 6. An increase 

 in nuclei has taken place in the outer wall of the process. Between it and the ectoderm 

 there is a slight concentration of mesenchyme, clearly derived from the same source as 

 the general mesenchyme. 



Simultaneously with these changes in the mesoderm, or possibly before them, 

 though of this I could not quite satisfy myself, the ectoderm shows an increase of nuclei 

 in this region (fig. 6). 



At a slightly later stage the position of the limb is marked by an external pro- 

 jection, due to an increased accumulation of mesenchyme cells beneath the ectoderm. 

 This is now considerably thickened here. It is irregularly two-layered. The middle 

 of the projection is opposite the myoseptum M1-M2. 



The ventral processes of M2 and M3, which take the chief part in muscularising the 

 pectoral fin, have at this stage shifted their positions relatively to their myotomes. 

 That of M2 has moved forwards so that it now lies wholly under Ml. It is in the 

 form of an elongated, moderately compact mesenchymatous mass. That of M3, though 

 remaining attached to its myotome, is much pulled forward, so that the end of it comes 

 in contact, and practically fuses, with the ventral process of M2 (cf. text figure 7), 

 at the level of myoseptum M1-M2 ; i.e. the ventral processes of both M2 and M3 

 reach a full myotome in front of the one to which they belong. The outer wall of 

 the ventral process M3 has broken up, but the inner wall is always recognisable as a 

 more or less definite epithelium. 



By far the greater number, if not all, of the mesoderm cells of the fin (apart from 

 the mesenchyme) are derived from M2 and M3. # At our present stage, though, the 

 ventral process of M4 is directed strongly forward, so as to reach up nearly to the front 

 end of M3, underlying the posterior part of the obliquely forward sloping ventral pro- 

 cess M3. The tip ends raggedly, as if cells were being given off to the fin rudiment, 

 but whether this is or is not the case could not be determined directly by observation. 

 However, the forward slope of this ventral process towards the fin rudiment (the ventral 

 processes of M5 and of the myotomes behind this slope backward) makes it probable 

 that it did contribute to the mesoderm of the fin at one time, though, if it still does, 

 the absence of a branch from N4 to the brachial plexus shows that its contribution 

 atrophies during ontogeny. 



We saw that at its first appearance the centre of the external projection forming the 

 rudiment of the fore-limb was opposite myoseptum M1-M2, — i.e. in front of the 



* The contribution of cells from M3 seems to be considerable, yet the nerve from sp. nerve 3 to the brachial 

 plexus disappears during development — p. 623. 



