CHONDROCRANIUM OF A 20 MM. HUMAN EMBRYO 625 



be followed from nasal tip to sphenoid body. The connective 

 tissue mentioned by Fawcett as covering in the nasal region at 

 the site of the future nasal bones and the frontal is present. 



Lateral and ventral to the nasal region, the maxillae are 

 beginning to form. As Fawcett states, there is one center of 

 ossification. Frontal, palatal, and alveolar processes can be 

 identified (fig. 9, A). 



In the course of the description of my own reconstruction, I 

 have dealt with the differences as compared to the reconstruc- 

 tions of Levi and Macklin. 



Von Noorden reconstructed 18.5 mm. and 23 mm. embryos. 

 It is to be regretted that his descriptions and illustrations are 

 not very complete. As compared to the 18.5 mm. embryo 

 described by him, the following advances may be noted in the 

 20 mm. stage: 



1. Formation of foramen rotundum, and outline of foramen 

 ovale. 2. Formation of carotid canal by the extension of the 

 commissura ali-cochlearis to the ventral pole of the cochlea. 

 3. Appearance of mastoid processes. 4. Joining of the parietal 

 plates to form the tectum posterius. 5. Appearance of maxillae. 



Von Noorden states that the optic foramina are formed in his 

 embryo. There is no indication of them in mine. 



As compared to Jacoby's 30 mm. embryo, my reconstruction 

 shows (1) much less development of the nasal region. In Jacoby's 

 reconstruction there are well-developed side-walls which are con- 

 nected with the septum to form the nasal roof. (2) Lack of 

 cartilaginous connection between alae orbitales and pre-sphenoid 

 and ethmoid regions. Examination of Jacoby's model shows 

 that the alae orbitales have three connections to neighboring 

 cartilaginous structures, to wit : to the pre-sphenoid through the 

 taenia post-optica, to the interorbital septum through the pro- 

 optica, and to the ect-ethmoid through the cartilago spheno- 

 ethmoidalis. All three of these processes are undeveloped in 

 my embryo. This accounts for the absence of the optic foramen 

 and the fissura orbito-nasalis. Another extension of the ala 

 orbitalis, dorsal and caudal toward the parietal plate, present in 

 Jacoby's embryo, is absent in mine. Briefly, at this stage all the 



