SEX-CELLS OF AMIA AND LEPIDOSTEUS 13 
yolk spherules is then probably associated with the difference 
in the size of the cells. The peripheral entodermal layer which 
we have designated as division three is interrupted latero- 
ventrally by blood vessels lying in the mesoderm. 
The lateral plates of mesoderm have long since broken away 
from the mesoblastic somites. Their inner margins lie at some 
distance to each side of the median line. While there is the 
slightest tendency in places for the splanchnic and somatic layers 
of mesoderm to split apart along the medial margins of the lat- 
eral plates, the remainder of the lateral plates show no indication 
of a splitting, even in the arrangement of the nuclei. It is, how- 
ever, quite probable that such a plane of cleavage is already laid 
down. This is shown by the sex-cells (text fig. A) being imbed- 
ded in the lateral plates. When the somatopleure and splanchno- 
pleure separate later, these will be found to lie in the coelomic 
cavity, being for a time merely adherent to the coelomic sur- 
face of the medial portion of the somatic mesoderm. One can 
fairly assume that during the period of migration, represented by 
fig. 5, the sex-cells push their way between the two layers of 
mesoderm following the potential cleft that separates them. 
Text fig. A is very suggestive, as it shows sex-cells situated at 
intervals from a point just beyond that at which the roof and 
floor entoderm join the peripheral entoderm. The path of their 
migration is thus clearly marked out. In this figure it should be 
noted that the most laterally situated sex-cell lies in the ento- 
derm, while all of the others are clearly imbedded in the lateral 
plates of mesoderm as already indicated. In but one or two of 
the many specimens examined were there any sex-cells found in 
the roof or gut entoderm. They arise in the peripheral entoderm 
from which they migrate into the lateral plates of mesoderm and 
through them to their medial borders, whence, as I shall later 
show, they pass into the sex-gland anlagen after the formation of 
the coelomic cavity. 
The total number of sex-cells found in the mesoderm of the 
specimen of this stage was 87. Of these 40 were found on the 
right side and 47 on the left. Text fig. A will indicate their dis- 
tribution. 
