The pronephros of Scyllium canicula. 229 
cleared for about three hours in cedar-wood oil, and then transferred 
to paraffin (b, p. 52° C). The sections were placed upon slides and 
the paraffin removed by xylol, after which they were mounted direct 
in Canada Balsam, or further stained with DELAFIELD’s Haematoxylin 
and Eosin, or Orange G in Absolute Alcohol before finally clearing 
and mounting. 
The method adopted in studying the pronephros at different 
stages was as follows: for any particular stage that series of sections 
was selected which, by comparison with other series at about the 
same age, showed the most typical condition of the pronephros. The 
sections throughout the whole of the pronephric region were then 
microphotographed, and prints prepared and correctly arranged in 
order and numbered. In this way I obtained a set of serial photo- 
graphs of the pronephros at different stages in its condition, and 
thus I was able to examine the structure more conveniently than 
by an examination of the sections themselves only. In order to 
facilitate and shorten the description of transverse sections of the 
embryos below, the number of slides used for mounting any parti- 
cular embryo has been stated, and also the number of sections on 
each slide. Thus section 4,33 would be the thirty-third section on 
the fourth slide, i. e. the number before the comma refers to the 
slide, and the number after the comma to the section on the slide. 
In the case of each embryo described a table is given showing 
the extent of the anterior segments on each side of the body. 
Embryo A. 
3'/, mm long. Fixed in corrosive acetic for fifteen minutes. 
Stained in Boraxcarmine for fourty-eight hours. Mounted upon two 
slides having 158 and 142 sections respectively. Sections of this 
embryo showed that the pronephric rudiment had not made its appea- 
rance, at least, not sufficiently as to be indubitably recognized. 
The differentiation of the somatic mesoderm layer had not pro- 
ceeded very far at this stage (slightly younger than Embryo B), 
although there were indications in some sections through the region 
of segments VII—X that the “primitive kidney groove” was be- 
coming distinct. 
This stage may fairly be taken as a starting point in con- 
sidering the development of the pronephros and its duct in Seyllium 
canicula. No gill-pouches have yet appeared. 
Plate 18 Fig. 1 is a photograph of a section through the future 
157 
