THE FORMATION OF THE LAYERS IN AMPHIOXUS. 291 
picrosulphuric acid and in corrosive sublimate and acetic acid, 
but for all later stages osmic acid material alone has been used. 
Samassa and I simultaneously employed the method of 
double-embedding in celloidin and paraffin, which, indeed, is 
the only method of dealing effectively with such minute and 
delicate embryos. As modified by me the procedure was as 
follows : The celloidin containing the embi’yos after being 
congealed in chloroform, was transferred to cedar oil. In 
this oil it became as clear as glass, so that the embedded 
embryo could be examined under the microscope and its 
orientation determined. An appropriately shaped piece of 
celloidin was then cut out and embedded in paraffin. The 
sections were stained on the slide. My conclusions so far as 
the gastrulatiou is concerned are based only on sagittal 
sections, the orientation of which was determined as I have 
described, but the accuracy of this orientation was tested by 
counting the sections in the series and observing whether the 
extreme ones at either side were alike in form. This is pos- 
sible only where the direction of the sections is accurately 
sagittal. The median section in each series was then figured, 
and all the figures are drawn with about the same magnifi- 
cation, viz. 900-1000 diameters. 
The Gastrulation. 
AVe shall commence our study of the development of 
Amphioxus with the stage of the flattened blastula, for in 
this stage the axes of symmetry can for the first time be 
determined. A median sagittal section through an embi’yo 
of this stage is represented in PI. 1, fig. 1. The outline is 
hemispherical, and for reasons which will presently emerge 
the flat surface is dii’ected backwards. It will be observed 
that the cells on the convex side are somewhat rectangular 
in outline and much smaller than the tall columnar cells on 
the flat side. In the upper part of the section where the 
convex part of the embryo abuts on the flat side there is an 
abrupt passage from the rectangular to the columnar type of 
