PRIMORDIAL CRANIUM OF THE CAT 287 



method. These are of great value for studies of the cartilaginous 

 skeleton, especially when used in comparison mth the sections. 

 Cat embryos were fixed in formalin, stained with methylene 

 blue, decolorized, cleared and preserved in hquid Canada bal- 

 sam. The last procedure, which departs shghtly from van 

 Wijhe's final treatment, is of much advantage in permitting the 

 free handling and turning of the specimen so that it may be 

 easily observed from every aspect. The following stages were 

 prepared and studied: Cat embryos of 10 nun., 12 mm., 15 mm.^ 

 17 mm., 20 nmi., 24 mm., 30 mm., and 35 mm. 



As stated above, the present paper centers upon a particular 

 stage, with which earlier and later stages have been compared. 

 In accordance with this method the structures of each region 

 of the chondrocranium as found in the stage of 23.1 mm., that 

 is, the stage of the model, are first described, then younger stages 

 are considered. 



PART I. OBSERVATIONS 



Basal plate. — Embryos of 23.1 mm. 



The term basal plate is used here to designate that portion 

 of the floor of the chondrocranium corresponding in cephalo- 

 caudal extent with the cranial part of the notochord, and in 

 lateral extent to the level of the basal foramina and cochlear 

 capsule. 



The basal plate in the embryo of 23.1 mm. is represented in 

 figures 1, 2 and 3. In the otic region it is very narrow; in the 

 occipital region, on the contrary, very broad. Foramina are 

 present at the margin of the basal plate, between it and the 

 cranial side walls, but no openings exist in the plate itself. 

 These marginal foramina are the fissura basi-cochlearis posterior^ 

 the jugular and hypoglossal foramina. 



The anterior, or otic portion of the basal plate, subcyhndrical 

 in shape, Ues between the cochlear prominences, and, within 



