Observations on the development of the excretory system in Turtles. 685 



Stage VII is 8 mm long and the head is bent. Stage VIII is 9 mm 

 long. The head and the tail are bent. Stage IX shows the first trace 

 of pigment in the eye. Stage XVI has a carapace 7 mm long and a 

 marked neck. The body is thick, the limbs slightly bent and the feet 

 show the first signs of the formation of toes. The head is more than 

 half as large as the body and the eyes are very large. 



Individuals differ somewhat from the typical condition, espe- 

 cially in the younger stages. For here as in other forms, the size, 

 particularly the length, does not accurately indicate the development 

 of the organs, which varies considerably in individuals of apparently 

 the same age. The stages are numbered with the Roman numerals 

 beginning with the youngest. There are many gaps much greater 

 than I could wish, chiefly in the younger stages. I have used the 

 Flatypeltis embryos almost exclusively in this study. 



Researches. 



In the study of the origin of the pronephros and of the pro- 

 nephric duct, the embryo of the shark has been considered most 

 valuable on account of the ancestral position of the shark in the 

 Vertebrate phylum, and has been the object of much research. The 

 turtle, also, as the one of the oldest of the Amniotes, is a very inter- 

 esting form for the study of this subject. It should be good ground 

 for solving the theoretical problem as to whether the entire excretory 

 system, embryonic and adult, consists of three parts of one organ, or 

 whether there are really three distinct organs, as well as the questions 

 of origin which are involved in such a study. 



In the present discussion I shall give first the results of ray 

 own observations, following the order of development, and beginning 

 with the pronephros. A comparison of my results with those of other 

 workers and the theoretical conclusions suggested by this study will 

 follow. 



Pronephros. 



Stage II. 



The basis of the pronephros in turtles is seen in my youngest 

 embryos of stage IL It arises as solid outgrowths, at first like swell- 

 ings, in the somatic region at the posterior end of the somites. These 

 grow backward and overlie those following with which they soon fuse. 

 The fourth somite (behind the ear) and the tenth seem to be the 



