62 



SKULL OF A 43-MM. HUMAN FETUS. 



missure, which curves upward and forward around 

 the canalicular part of the otic capsule, to become 

 directly continuous with the capsulo-parietal com- 

 missure. It is interrupted by the conspicuous 

 capsulo-occipital foramen (figs. 5 and 6). Figure 14 

 shows the surface where the commissure has been 

 severed. 



OCCIPITO-PARIETAL GROOVE. 



The next section of the boundary is formed by 

 the occipito-parietal commissure, which meets the 

 preceding at an acute angle. It joins together the 

 squama occipitalis and parietal plate. The latter 

 bends inward a little upon the former to make the 

 shallow occipito-parietal groove (fig. 14), which 

 marks the position of the commissure upon the 

 ental surface. The sections show that the car- 

 tilage is very much thinner here, especially at the 

 dorsal end, though not materially different in 

 quality as compared with that above and below. 

 As in la, there is no trace of a corresponding groove 

 upon the ectal surface. 



The ventral end of this groove is not very well 

 defined, but may be placed just above the capsulo- 

 occipital foramen. On the right side there is here 

 a very small foramen. The dorsal limit of the 

 groove is marked by the conspicuous occipito- 

 parietal notch which separates the pointed dorso- 

 niedial termination of the parietal plate from the 

 underlying supraoccipital element (fig. 14). Be- 

 tween these extremities the groove pursues a 

 course almost directly backward, upward, and 

 inward. 



The occipito-parietal groove has been noted by 

 Kernan (1916) as early as the 20 mm. stage. In 

 7a it was well marked and presented two foramina 

 upon the right side and one upon the left. These 

 foramina are not represented in No. 886, nor are 

 they noted in No. 460 or in Kernan's specimen. 

 Furthermore, in 7a the occipito-parietal notch was 

 considerably deeper and, indeed, terminated as a 

 narrow slit. What appeared to be degenerating 

 cartilage cells were found in 7a on the right side 

 (the sections did not include the corresponding 

 region on the left side), uniting the parietal plate 

 witli the squama in the region of this cleft. From 

 this it would seem that there is a gradual disunion 

 of these two plates in progress at the posterior end 

 of the occipito-parietal commissure during this 

 developmental period. 



The groove for the endolymphatic sac, which 

 Lewis mentions having found upon the "masto- 

 squamal plate," corresponds to the anterior end of 

 the occipito-parietal groove. In No. 886 the 

 attenuated prolongation from the endolymphatic 

 sac lies immediately medial to the anterior end of 



the groove, but separated from it by 0.5 mm. 

 In 7a it was similarly situated. It is not im- 

 possible that this groove may be related histori- 

 cally to the endolymphatic sac, for it leads back- 

 ward and inward to a point quite close to the pro- 

 cessus ascendens; and in the lizard, as has been 

 pointed out by Gaupp (1900) and others, the 

 endolymphatic sac lies just lateral to the pro- 

 cessus ascendens. The region between the otic 

 capsule and the tectum posterius has undergone 

 an enormous amount of extension in the mammals, 

 and particularly in man, and the endolymphatic 

 sac has accordingly become removed from its 

 original position above the tectum posterius, 

 because of its connection with the otic capsule. 



PROCESSUS ASCENDENS. 



Of the cranial boundary there remains to be 

 completed only that part lying between the afore- 

 mentioned occipito-parietal notches. This line, 

 which marks the thin upper border of the supra- 

 occipital cartilage, is transverse, with a backward 

 bowing (fig. 1). Projecting upward from this, in 

 the midline, is a single small nodule of rather young 

 cartilage. Upon either side of it are the funda- 

 ments of the interparietal bone, which are seen as 

 thin strips lying close to the upper edge of the 

 supraoccipital. This nodule is spheroidal and its 

 anterior face projects forward a little. It is 

 directly continuous below with the supraoccipital 

 cartilage. This process corresponds to the "short 

 process" of Fawcett (19106), which he found in a 

 30 mm. human embryo. He states that it "seems 

 to correspond exactly with the ascending process 

 of the tectum synoticum of reptiles and amphibia 

 as figured in Hertwig's Handbook of Embryology." 

 In this homologization I agree. Bolk (1904) 

 shows such a process in his figure 1, plate 6, pro- 

 jecting upward from the "Knorpelspange," but 

 does not describe or name it. This figure was 

 drawn from a human skull, apparently of about 

 the same stage of development as No. 886. The 

 "Knorpelspange" of Bolk, be it remarked, is 

 merely the upper edge of the tectum posterius, 

 which remains uncalcified, and so may be stained 

 by the methylene blue of the van Wihje method 

 which he uses, while the calcified cartilage below 

 it, which Bolk erroneously interprets as membrane, 

 is not stained in his preparations. 



In the pig's chondrocranium Mead (1909) has 

 described a single free nodule of cartilage just above 

 the tectum posterius in the midline, which seems 

 to correspond to the processus ascendens of No. 

 886. He thought it might possibly be the homo- 

 logue of the processus ascendens of the tectum 



