18 WILLIAM MORTON WHEELER, 



short constricted portion indicated at z in the transverse and frontal 

 sections. 



b) The first Stages in the Development of the 

 Pronephros. 

 Towards the end of stage 2 and the beginning of stage 3 sections 

 through the middle of a segment just behind the branchial region 

 have the appearance of Fig. 9, PI. 1. The myotome is more ad- 

 vanced, for its cavity has disappeared, and the cells of the muscle- 

 plate impl) are flattening out and crowding together. The lateral 

 widening of the mesoderm is now sharply cut off from the myotome 

 and the nephrocœle has enlarged owing to an accumulation of 

 liquid between the somatic and splanchnic layers. In the embryo 

 from which the section is taken I find this condition in three suc- 

 cessive segments, each of which will give rise to a pronephric tubule. 

 The next stage is shown in Fig. 38, a section which passes through 

 what I regard as the second or third pronephric tubule. The neck 

 of the embryo is cut in such a way that the foregut appears as a 

 closed, thick-walled tube. The myotome is joined to the lateral 

 mesoderm by means of a very poorly developed "intermediate plate" 

 at z. The cells of the muscle-plate {mpl) are much flattened and 

 there are no traces of a distinct myocœle except as a hne separating 

 the cutis-plate {cpT) from the myogenic layer. The nephrocœle, on 

 the other hand, is very large and distinct, and the thickened somatic 

 layer which forms its outer wall projects upwards (pron) as a point 

 containing a prominent karyokinetic figure. This section is hardly so 

 typical as the one represented in Fig. 39, which is also from an 

 embryo in stage 3. Here the myotome is distinctly separated from 

 the remaining mesoderm and prolonged at its inner lower corner into 

 a hollow pointed diverticulum, the sclerotome. The pronephric tubule 

 (pron), which is the fourth in the series, is of a more cylindrical 

 shape than the tubule in Fig. 38. The myocœle lies in its base. The 

 two layers of the mesoderm ventral to the pronephric tubule are 

 closely applied to each other and not always clearly distinguishable. 

 The relations of the pronephric tubules to their respective segments 

 are best seen in sagittal (Fig. 27) and frontal sections (Fig. 26), In 

 Fig. 27 the whole pronephros is included in a single section and six 

 distinct tubules (pron^ to pron^) are clearly shown. The most an- 

 terior (pron'^) has a wide lumen which communicates with the nephro- 

 cœle (cœ) and is turned slightly forward. The typical condition is 



