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 corpuscles. 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 cartilag- 

 inous roof the skull, there is a space largely filled with blood 

 corpuscles, and with a few strands of reticular connective tissue. 

 A few scattered 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 grows over the dorsum, it is easy to see how sonic 

 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 (pi. 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- 



