4 F. L. LANDACRE 
and seems to encroach upon the territory occupied by the ecto- 
dermic evagination forming the placode; at least the placode is 
so closely applied to the posterior surface of the pharyngeal pocket 
that it is often difficult and sometimes impossible to tell where one 
ends and the other begins. Aside from this feature the condi- 
tions are quite similar to those found in Ameturus with the excep- 
tion that the visceral portion of the IX is not purely placodal in 
origin and that the placode of the VII nerve is much larger in 
Lepidosteus than in Ameiurus. In view of these facts I shall 
describe the placode of the VII nerve in detail, and treat the re- 
maining placodes briefly. 
MATERIAL 
The material consists of thirty-three stages taken at intervals 
of six hours from one lot of eggs. Usually this would be too long: 
an interval to follow accurately the changes in the cerebral gan- 
glia but, by cutting a large number of series of any given age, it 
is rare that one cannot pick out some one of a given series that 
is as far advanced or even further advanced than the youngest 
stage of the next older series; so that if the series are sufficiently 
numerous they become practically continuous. The sections 
were cut 6 uw thick and stained in bulk in Delafield’s haematoxylin 
one-sixth the strength of the stock solution, for twenty-four hours. 
This gave a much better differentiated stain than when sections 
were stained on the slide and owing to the amount of yolk gran- 
ules present is much superior to Heidenhain’s stain. 
Table 1 shows the age, length and increments in age and length 
of embryos of Lepidosteus osseus ranging from 100 hours after 
fertilization to 272 hours after fertilization. Several series 
younger than 100 hours are referred to in the body of the paper 
but are not included in the table because they had not been freed 
from the membranes before fixation and consequently could not 
be measured accurately. They are referred to by age only. 
