64 



SKULL OF A 43-MM. HUMAN FETUS. 



orly. This steep area corresponds to the deep 

 condylar fossa of the ectal surface. The recess is 

 partially overhung anteriorly by the conspicuous 

 posterior ampullary prominence, which gives to 

 the region a cave-like appearance. 



As in la there is on the left side, but not on the 

 right, a small paracondyloid foramen (200 micra 

 in antero-posterior diameter and rather less in 

 transverse) which pierces the thin lamina alaris 

 just lateral to the jugular tubercle. It contains 

 only connective tissue. The corresponding area 

 on the right side is very thin. 



Paracondyloid process. — The paracondyloid pro- 

 cess, less stout than in 7a, is a prominent object in 

 figure 2, appearing as a ridge passing outward and 

 slightly upward and forward from the condyle, 

 and becoming more sharply marked laterally. It 

 ends in a point which projects outward and a little 

 downward (fig. 10), and which lies just lateral to 

 and below the outer limit of the jugular foramen. 

 I have already expressed myself (Macklin, 1914) 

 as agreeing with Levi (1900) and Voit (1909) in 

 their identification of the corresponding structure 

 of their specimens as the forerunner of the jugular 

 process of the occipital bone, and Lewis (1920) is 

 in accord with this idea. Kernan (1916, p. 626) 

 asserts that this process unde goes absorption. 



It is at the transverse ridge formed by the con- 

 dylar and paracondyloid processes that the 

 occipital cartilage makes its sharp bend upward, as 

 is seen in side views of the skull (figs. 5, 6, 10). In 

 this way the ectal surface is divided into anterior 

 (or basal) and posterior (or nuchal) areas. It is 

 of interest that, at this stage, the paracondyloid 

 process reaches laterally far beyond the joined 

 transverse and costal processes of the underlying 

 atlas, as is seen well in figure 4. This contrasts 

 most strongly with the condition in the adult, 

 where the reverse relation holds, the transverse 

 process of the atlas overreaching the jugular 

 process. 



Kernan describes the tip of the paracondyloid 

 process in his 20 mm. human embryo as separate 

 from the cartilage medial to it, but attached to the 

 basioccipital through a thin process of cartilage 

 which passes medially and cranially in front of the 

 hypoglossal foramen. This "thin process" (which 

 is what I have described as the posterior border of 

 the jugular foramen, and which is bounded 

 behind, on the left side, by the paracondyloid 

 foramen) he represents to be the independent 

 costal process of the second occipital vertebra (not 

 of the first, as I suggested), and the paracondyloid 

 process is considered to be the caudal end of this. 

 Thus, according to Kernan, the paracondyloid 



process does not represent the transverse process 

 (and possibly the costal process) of the occipital 

 vertebra, as I suggested (Macklin 1914). How- 

 ever, it should be noted that the paracondyloid 

 process of Kernan is but the lateral free tip of the 

 structure which I described under the same name. 

 In Kernan's 20 mm. stage the paracondyloid 

 process is very small as compared with its con- 

 dition in No. 886 and 7a. Even if the tip of it 

 does belong to the second vertebra, it would still 

 seem that the main mass of it is to be looked upon 

 as having been derived from the transverse and 

 possibly the costal process of the occipital verte- 

 bra. Kernan notes that there is no independent 

 costal element of the occipital vertebra. 



The paracondyloid process of Kernan's speci- 

 men is directly continuous above with the lateral 

 portion of the lamina alaris, which appears upon 

 the ectal surface as the lower end of the formation 

 which I have termed the crescentic ridge. Kernan 

 erroneously speaks of this as "the inferior nuchal 

 line of the ex-occipital portion of the adult bone." 

 This lateral portion of the lamina alaris shows, 

 according to Kernan, alternating interruptions 

 and junctions with the cartilage lying medial to it 

 and is looked upon as a costal bar, the junctions 

 representing the bases of transverse processes of 

 vertebrae. He suggests that "in the lamina alaris 

 are represented the costal and transverse processes 

 of several vertebrae. The outer bar represents the 

 fused costal elements and may be termed the 

 costal bar." He claims to be able to discern 

 parts of three vertebra? in the lamina alaris. 



Lewis, who describes a stage of the human 

 embryo (21 mm.) ahnost identical with that of 

 Kernan, says nothing as to this complicated 

 structure of the lamina alaris nor as to the inde- 

 pendence of the tip of the paracondyloid process. 

 He states (p. 317): "The occipital transverse 

 process forms part of the caudal and lateral 

 margins of the jugular foramen and continues up 

 into the squama and alar lamina without line of 

 demarcation." Further (p. 318) he says: "The 

 transverse or jugular process springs from the 

 occipital hemiareh at the junction of the roots and 

 lamina and projects laterally back of the jugular 

 foramen. We have already noted its serial 

 relationship with the vertebral transverse proc- 

 esses. The lateral extremity of the jugular 

 process has a knob-like enlargement, and into this 

 are inserted the rectus capitis lateralis muscle and 

 the occipito-mastoid muscle." 



It should be remarked that the structure which 

 Kernan in his plate 3 labels as "Processus costalis 

 et transversus, occipital vertebra" is labeled by 



