DEVELOPMENT OF NEPHKIDIA OF PHERETIMA 97 



in the same way and opened by a mid- ventral incision; 

 the albumen is removed but not the endoderm : these, when 

 mounted flat, show the septal nephridia in all stages of develop- 

 ment, as represented in figs. 14 and 15. Since they he in two 

 rows, one on each side of the dorsal vessel, it is best to open the 

 embryos from the ventral side and look for the nephridia on 

 each side of the dorsal vessel. In order to avoid any displace- 

 ment of the septal nephridia it is best to leave the endoderm 

 on ; but it is necessary to brush carefully the inside of the 

 embryo with a fine camel-hair brush, so that all yolk-material 

 sticking to the inside of the gut is removed and the prepara- 

 tion rendered quite transparent to show the septal nephridia 

 to the best advantage. 



The most difficult part of the task, however, was to trace the 

 initial stages of development of the septal nephridia — to find 

 out whether the ' rudiment ' ('Anlage ') of the septal nephridia 

 arose as a multiplication of one or more cells belonging to the 

 walls of the adjoining coelomic sacs (the septa), or it was 

 formed by a group of cells between the two contiguous sheets 

 of the intersegmental septa. For this purpose it was necessary 

 to have serial longitudinal sections of the dorsal and dorso- 

 lateral parts of an embryo of suitable age, i.e. one in which one 

 could expect to find the septal nephridia in very early stages 

 of development. The difficulty in getting such sections arises 

 from the fact that the gut in embryos of this age is so enor- 

 mously distended with food-yolk as to squeeze out of existence 

 altogether the coelomic cavity between the body-wall and 

 the gut dorsally. Consequently it becomes impossible to distin- 

 guish, in sections of such embryos, the septa and the nephridial 

 masses on them. Many series of sections were cut of embryos 

 which had been flattened out, after being cut open ventrally 

 and the food-yolk removed. In these series, although the 

 coelom could be distinguished in some of the segments, it was 

 obliterated in others, and no accurate and reliable observations 

 could be made. In many of the embryos part of the posterior 

 end was cut off before fixation to allow the food-yolk to ooze 

 out and thus let the wall of the gut shrink away from the 



