end. Below the paraseptal cartilages is a strap-like connec- 

 tive presumed to be formed from the ectochoanal processes 

 seen in other mammals. 



In its development this chondrocranium differs from those 

 of other mammals in that there is but one pair of occipital 

 arches involved, the most posterior one. The orbital carti- 

 lage arises as a separate center, which becomes connected 

 with the basis cranii through the pila antotica and only 

 later through the pila preoptica. 



The polar cartilages are in fact only the posterior ends of 

 the trabeculae, which gradually extend forward and fuse at 

 the midline to form the trabecula communis. The processus 

 alaris has a condensation of precartilage at its tip; this later 

 becomes incorporated into it and the line of fusion is indi- 

 cated by the alar canal. This transitory element is assumed 

 to represent the ala temporalis, but the fact that it is never 

 a distinct element denies this. 



The anterior transverse laminae extend out from the ven- 

 tral margin of the nasal septum to give rise to the marginal 

 process. The lateral wall of the capsule then contacts the 

 anterior transverse lamina to form the broad zona annularis. 

 The lateral wall is penetrated by a foramen, the incisure 

 infraconchae. This foramen is formed by ventral closure of 

 a notch in the side wall by forward growth of the processus 

 ma.xillaris anterior. 



The chondrocranium of Tachyglossus is quite similar. In its 

 development, the trabeculae show their bilateral origin more 

 clearly but still fuse to form an anterior trabecula communis. 

 The polar portions lie lateral to the internal carotids. A tec- 

 tum transversum appears, temporarilyjoining the anterior 

 parts of the parietal plates across the midline. Such a tectum 

 also appears on occasion in man. 



There is no evidence of an ala temporalis, and even the 

 precartilage condensation observed in the platypus is lack- 

 ing. The nasal septum arises from an independent center 

 above the trabecula communis. Throughout the nasal region 

 there are many separate chondrification centers. The dorsum 

 sellae is only slightly developed. 



Meckel's cartilages are the same as in the placentals; they 

 are partially fused at the symphysis. The hyoid arch and 

 branchial skeleton are similar to the placental's, but cerato- 

 brachials I and II remain more distinct and are not lost in 

 the thyroid cartilage. 



In summary, the monotreme chondrocranium is well 

 marked from that of the other mammals by the presence of a 

 common j-ugulohypoglossal foramen, a pila antotica, a large 

 olfactory fenestra, palatal processes extending posteromedi- 

 ally from the anterior transverse lamina, and marginal 

 processes extending laterally; also, the lack of a pila metop- 

 tica, a recognizable ala temporalis, and a posterior trans- 

 verse lamina for the nasal capsule. 



Development of the bony head skeleton 



Placental The first appearance of bone in the head region 



IS followed by the progressive appearance and development 

 of centers until the adult form is achieved. In the course of 

 development, additional bones may appear between the 

 typical bones. Such bones are more frequently observed 

 along the dorsal midline, particularly in the anterior or 

 bregmatic fontanelle between the paired frontals and pari- 

 etals. Another variation is the subdivision of typical bones 

 into two or more parts. For example, the parietals of some 

 Australian aborigines are divided into lateral and medial 

 parts. 



The first bones to appear are the dermal bones of the jaws, 

 then the anterior side wall of the brain case and palate, and 

 finally the posterior brain case (Figure 3-16). The endo- 

 chondral bones appear about midway in the dermal se- 

 quence, beginning with the occipitals, and ending with those 

 of the otic and nasal capsules. The sequence of appearance 

 shows some variation; irregular centers or subdivisions of 

 centers can occur. Dermal bones appear first as reticula 

 (networks) of fibers and chondral bones appear as granular 

 masses. In section the chondral bones are observed to involve 

 perichondral laminae and a network of endochondral fibers. 

 In the various placentals examined, there are some varia- 

 tions that should be noted. The postparietal fuses with the 

 supraoccipital, or the parietals; it may remain distinct from 

 both (Figures 3-1, 3-16). Two centers of origin are involved 

 in the parietal of man, one above the other, whereas in the 

 cat, rabbit, or rat only one center occurs. A similar situa- 

 tion is observed in the pig, where the exoccipital arises from 

 two main pairs of centers. In man the supraoccipital may 

 arise from two pairs of centers; the more lateral one has some- 

 times been identified as the tabular bone. The extreme of 

 variation is observed in the petrosal mass, which shows many 

 (three to thirteen) and sometimes irregularly positioned 

 centers. 



Two bones that present difficult situations for evaluation 

 are the alisphenoid and the orbitosphenoid, not only because 

 of variation in dermal ossification centers but also endo- 

 chondral centers. The alisphenoid by definition arises 

 through ossification of the ala temporalis, usually from one 

 center, although two are observed in the cat. Associated with 

 the chondral bone is a dermal plate derived by outgrowth 

 from the chondral center into the mesenchyme. Extension 

 of a chondral bone into adjacent mesenchyme occurs with 

 other bones as well — for example, the supraoccipital of some 

 insectivores extends beyond the cartilaginous precursor as 

 does also the orbitosphenoid. The alisphenoid may be 

 attached very early in its ossification to the basisphenoid 

 (rabbit) or it may remain separate for some time (man, cat). 

 The orbitosphenoid is even more variable in form and 

 origin. In the rat (Figure 3-17) there are the typical lateral 

 (orbital wing or outer part of pila metoptica) and medial 

 (in the trabecula or ala hypochiasmatica) centers and, in 

 addition, a small pila preoptica center. The trabecular cen- 

 ter appears first and fuses at the midline to form a distinct 

 "presphenoid." Then the lateral center appears, followed 



DEVELOPMENT OF THE HEAD SKELETON • 57 



