322 CARTILAGINOUS SKULL OF A 21 MM. HUMAN EMBRYO. 



suggests that the mastoid process may be regarded as a remnant of the proximal 

 end of a branchial cartilage, while its serial relation with the transverse process of 

 the occipital and cervical vertebrse, and its connection with the former by muscle, 

 as in embryo No. 460, or by ligament, as in embryos Nos. 431 and 382, suggest that 

 it may be a rudimentary portion of the transverse process of an occipital or temporal 

 vertebra. A still more speculative idea would be to consider the branchial bars as 

 serially related to the vertebral transverse process, and the occipito-mastoid muscle 

 could be looked upon as serially related to the intertransversarii muscles on the one 

 hand and to the stapedius on the other. 



THE ETHMOID CARTILAGE. 



The ethmoid cartilage consists of the mesethmoid and the lateral nasal cap- 

 sules joined to the former by precartilage. The ethmoid is very poorly developed 

 at this stage and all the figures include more or less precartilage. The gradual 

 transition from cartilage to young cartilage and precartilage makes it impossible 

 to draw sharp lines between the cartilage and precartilage. The mesethmoid 

 consists of fairly well-differentiated cartilage near its junction with the sphenoid. 

 In this region the pharyngeal edge is much thicker than the dorsal edge. It gradu- 

 ally tapers towards the apex, the entire septum attaining an even thickness. The 

 crista galli is relatively large when both cartilage and precartilage are included, 

 as shown in the various figures. 



The nasal capsules consist mostly of young cartilage and precartilage. They 

 are relatively simple and show only slight indications of the turbinate processes. 

 Part of the lateral surface enters into the formation of the medial wall of the orbit 

 and part is covered by maxillary and nasal blastema, the precartilaginous tips 

 extending beyond (fig. 15). The inner wall of the orbit is completed by frontal 

 blastema and precartilage of the orbital wing of the sphenoid. This blastema 

 and precartilage, as already noted, form a continuous sheet (fig. 3). The orbital 

 surface of the sheet is more or less closely united to the upper edge of the nasal 

 capsule (fig. 15). On the line of junction between the cartilage and frontal blastema 

 are found, as in the adult, the anterior and posterior ethmoidal foramina, through 

 which pass the nasociliary nerve and the ethmoidal branches. 



