. Text-book of embryology. Embryology. The jaw, composed of a closely set row of such columns, is sup- ported by the neighbouring parts of the epidermis which also under- go a certain amount of cornification. Thus just internal to the jaw is a cushion-like mass of large slightly cornified cells which forms an efficient backing to it (of. Fig. 45, A and B) while external to the jaw the surface, of the epidermis is composed ^ s> of flattened much cornified cells (Fig. 45, A). The oral combs consist of a pallisade- like arrangement of similar denticles which however in this case are not in con
. Text-book of embryology. Embryology. The jaw, composed of a closely set row of such columns, is sup- ported by the neighbouring parts of the epidermis which also under- go a certain amount of cornification. Thus just internal to the jaw is a cushion-like mass of large slightly cornified cells which forms an efficient backing to it (of. Fig. 45, A and B) while external to the jaw the surface, of the epidermis is composed ^ s> of flattened much cornified cells (Fig. 45, A). The oral combs consist of a pallisade- like arrangement of similar denticles which however in this case are not in contact. Fig. 45, C, shows a longitudinal section through the posterior labial comb of Palu- dicola. Here again we see a succession- column of epidermal cells commencing with a small initial cell near the inner sur- face of the epidermis. From the initial cell outwards the cells increase in size, become gradually cornified and each one fits closely into the base of the next one which be- comes more and more deeply excavated as the tip is approached. Two conspicuous differences distinguish the denticle of the oral comb from that of the jaw: (1) instead of being regularly conical in shape it is claw-shaped with serrated edges (Fig. 45, D) the tip being recurved, and (2) the hollow base of the cornified cell is not entirely occupied by its successor in the series: it also accommodates an indifferent cell of the epidermis (supporting cell of Gutzeit) which bulges into it. I have described the development of these interesting structures. Fig. 46.—Vertical section through lingual spine of Petromyzon. (After Warren, 1902.) A, earlier, B, later stage, d, dermis; , mesodermal papilla; s, functional spine; si, spine Please note that these images are extracted from scanned page images that may have been digitally enhanced for readability - coloration and appearance of these illustrations may not perfectly resemble the original MacBride, E. W. (Ernest William), 1866-194
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