32 BULLETIN : MUSEUM OF COMPARATIVE ZOOLOGY, 



I. Introduction. 



The present investigatious deal with the early development of the 

 mesonephros and of the Mtillerian duct. As these organs are in a measure 

 independent of each other in their development, the introductory remarks 

 on the Miillerian duct may best be left for the portion of the paper 

 dealing with the development of that organ. 



As is well known, the mesonephros consists essentially of a mass of 

 tubules, each of which, in its typical form, may be divided into three 

 regions: (1) a short duct leading from the body cavity into (2) a 

 dilatation which contains an invaginated vascular mass (the glomerulus) 

 and is in communication with the Wolffian duct by means of (3) a long, 

 coiled duct. These three parts may together be called a mesonephric 

 unit. According to their position, structure, and connections, the units 

 fall into various sets. In the higher Amphibia two chief sets can be 

 distinguished : a ventral set, in which the units (primary units) connect 

 directly with the Wolffian duct, and a smaller or greater number of dorsal 

 sets, which form connections with the Wolffian duct indirectly by open- 

 ing into the primary set. The dorsal sets are arranged in longitudinal 

 rows above the ventral set, and are designated as secondary, tertiary, etc. 



Among the problems presented by the development of the mesonephros 

 in Amphibia three stand out at present as of greater importance than 

 the others ; it is to these three that I have given most attention. They 

 are: (1) the origin of the tissue which forms the fundaments of the meso- 

 nephric units, (2) tlie origin of the dorsal sets of units, and (3) the 

 dysmetameric condition of the primary units, — supposing them to have 

 been oinginally metamerically arranged. 



Before considering the views that have been held in regard to the 

 origin of the mesonephric units, it is necessary to have a clear under- 

 standing of the condition of the mesoderm (mesothelinra) at the time 

 when they make their appearance. The mesoderm on either side of the 

 body presents two layers, the splanchnoderra ^ mesially (ventrally) and 

 the somatoderm ^ laterally (dorsally). These layers are separated by the 



1 I have coined these two words to fill a real need, the expressions " splanchnic 

 layer of the mesoderm " and " somatic layer of the mesoderm " being awkward 

 terms with whicli to designate these layers as compared with single words. The 

 terms " splanchnopleure " and " somatopleure " have been used in this sense, but 

 incorrectly, since the latter properly includes ectoderm as well as mesodermi'and 

 the former entoderm as well as mesoderm. 



