94 University of California Publications in Zoology 
beneath the skin in the very young specimens in which the 
cartilaginous roof is not yet developed. 
In the 18 mm. specimen was found the first indication of 
any tissue intervening between the brain covering and the roof 
of the skull, in the form of a blood sinus, a space across the 
dorsum of the myelencephalon filled with blood corpuseles. In 
the 21.5 and 22 mm. stages the condition is very similar except 
that the blood sinus is larger and more conspicuous. <A typical 
section through the myelencephalon of the 21.5 mm. embryo is 
shown in plate 12, figure 3. Between the band of columnar 
endothelial cells covering the fourth ventricle, and the eartilag- 
inous roof the skull, there is a space largely filled with blood 
corpuscles, and with a few strands of reticular connective tissue. 
A few seattered pigment cells may also be seen. Immediately 
beneath the skin, even in those embryos where the roof of the 
skull is not yet developed, there is a dense layer of pigment cells. 
As the cartilage srows over the dorsum, it is easy to see how some 
of the pigment cells might be pinched off and left inside, there 
to multiply and cause the apparently useless pigmentation of 
the covering of the brain. At any rate, it seems highly probable 
that the pigment cells found there are derived from the pig- 
mented layer of the integument. 
The next embryo available for study was a 55 mm. specimen 
which was cut in sections 10 microns in thickness and stained 
with Delafield’s haemotoxylin and erythrosin. The condition 
there presented is extremely instructive, as it is In every way 
intermediate between the 22 mm. stage and the adult form. The 
space between the covering of the fourth ventricle and the skull 
is much widened, being at least as wide as the depth of the 
fourth ventricle. This space is largely empty, but is partially 
filled by a reticulum of connective tissue (pl. 12, fig. 4). In 
this reticulum are large blood sinuses, more or less densely 
crowded with corpuscles, and with a number of large, scattered 
pigment cells. The latter differ from those in the adult structure 
in that they are far more regular in outline, often nearly round, 
and without the dendritic branches displayed later. It will be 
noticed that in plate 12, figure 4, there are lateral outpocketings 
of the ependymal epithelium. Farther caudad these outpocket- 
