HALL: iMESONEPHROS AND MULLERIAN DUCT IN AMPHIBIA. 77 



the cloaca. The development of the Miillerian duct shows that the 

 organ was originally more complex, for a portion of its fundament degen- 

 erates completely. In accordance with this fact, it is not surprising that 

 the earlier stages are quite variable. This variability has made it seem 

 best to describe individual larvae, — a rather cumbersome way, but one 

 which obviates the necessity of constantly mentioning exceptions. To 

 a certain extent, the larvae successively described represent successive 

 stages ; that is, they are so arranged that each one shows, on the iclwle, 

 an advance in oviducal development over the preceding one, although 

 in some particulars it may be less advanced. 



Larva I, 23 mm. 



In order to understand the early development of the Mtillerian duct, 

 it is necessary to follow the successive changes in the body cavity. 



In a larva in which no sign of the duct is yet present, a cross-section 

 cutting the anterior portion of the pronephros shows no body cavity, 

 the entire region being filled with loose mesenchyraatous tissue. A 

 section a little farther back, passing through the first nephrostome 

 (Fig. 31, Plate 3), shows three divisions of the body cavity on each 

 side of the body : (1) A ventral division (coel. v.) at the side of the 

 pericardium (pi'cr.) ; (2) a dorsal division, the glomerular cavity 

 (cav. gJm.), into which the nephrostomes open and the glomus pro- 

 trudes ; and (3) a cavity between these two, which I shall call the sub- 

 glomerular cavity (cav. sb'glm.). At the stage represented by the larva 

 under consideration, the sub-glomerular cavity is recognizable for only 

 two or three sections. It is really but a shallow anterior outpocketing 

 of the glomerular cavity, or, from another point of view, it may be said 

 that there is a narrow shelf of tissue (tab.) which juts out from the 

 anterior wall of the glomerular cavity, extending from the alimentary 

 tract on the median side to a region just ventral to the pronephros on 

 the lateral side. For convenience, I shall refer to this tissue simply as 

 the ^' shelf ." Following the sections caudad, one finds that the glom- 

 erular and sub-glomerular cavities soon become confluent. The single 

 cavity thus formed remains distinct from the ventral body cavity as far 

 back as a point posterior to the pronephros. Figure 37 (Plate 3) taken 

 from a section passing through the posterior part of the glomus on one 

 side of the body (the right, in the figure left) and through the second 

 nephrostome on the other, shows the dorsal (coel. d.) and ventral por- 

 tions of the body cavity (coel. v.) separated by tissue joining the lung 

 to the body wall (pn. -\-par.). 



VOL. XLV. — NO. 2 4 



