OCR Text |
Show 158 ON JACOBSON'S ORGAN IN THE CROCODILIA. [Feb. 1 7, those already described (ante, p. 151) concerning its relationship the vomerine ligament (lg., fig. 4), show it to be a secondary outgrowth, arising in correlation with the shortening up of the vomer, and having little, if anything, to do with the palatine process as ordinarily understood. VI. Parker, in his monograph on the development of the skull in the Crocodilia, described I the early differentiation of a basi-mandi-bular cartilage, such as he had previously encountered 2 in the embryo of Chelone viridis. H e states, on the authority of Prof. C. Stewart, of the R. College of Surgeons, that in the embryo Crocodile the conjugated distal ends of the mandibles (Meckel's cartilages) become dilated. My friend Mr. Boulenger has recently called attention in these ' Proceedings'3 to the existence of a small bone in the mandibular symphysis of Lleloderma; and he inclines towards associating it with the mento-Meckelian bones, well known to occur in the living Anura. The latter (m.m., fig. 8) arise as ossifications of the Meckelian cartilages, and the distal ends of those rods generally unite, in these animals, to form a prominent mass which may exactly correspond to the symphysial cartilage of Stewart. I have long been familiar with the fact that in some Anura (ex. Hyla ccerulea, fig. 8) this (m.b.) may become segmented off in the manner of the basal cartilages of the postGral visceral arches of the lower lchthyopsida, and of the cartilage described by Parker in the young embryos of Crocodilus and Chelone; it would therefore appear to be altogether independent of the mento-Meckelian bones. The existence of this " basi-mandibular " element of the mandibular arch is not without interest, in view of the belief in the serial homology between the latter and the postoral skeletal arches *. During this investigation I have had the good fortune to have been repeatedly in conference with m y friend Mr. Boulenger ; and my best thanks are due to him for having, by his valuable assistance and advice, rendered m y task, in itself pleasurable, doubly so. EXPLANATION OF PLATE XIV. Fig. 1. Crocodilus •palustris. Median longitudinal section through nasal region of dried skull. One-third nat. si~c. Fig. 2. Caiman niger, dried skull. A similar section to fig. 1, cut to the right of the middle line. One-third nat.. sige. Fig. 3. Lepus cuniculus. Dissection of inner portion of olfactory capsule of left side ; to show the vomer and other supporting elements, in relation to the organ of Jacobson. Twice nat. si~e. 1 Trans. Z. S. vol. xi. p. 280 (1883). 2 Challenger Rep. Zool. vol. i. pt. 5, p. 20 (1880). 3 Above, p. 111. 4 Cf. Huxley, Journ. Anat. and Phys. vol. x. pp. 421 & 427 (1S76); and Parker, op. cit. White has recently recorded (he existence of a basi-mandibular cartilage in Icemaryvs (Anat. Anz. 1890, p. 260). |