Fig. 77. Embryos of the common Frog. (After Remak.)
A. Young stage represented enclosed in the egg-membrane. The medullary plate is distinctly formed, but no part of the medullary canal is closed. bl. blastopore.
B. Older embryo after the closure of the medullary canal. oc. optic vesicle. Behind the optic vesicle are seen two visceral arches.
Anura. The pyriform medullary plate, already described, is the first external indication of the embryo. This plate appears about the stage represented in longitudinal section in [fig. 71] B. The feature most conspicuous in it at first is the axial groove. It soon becomes more prominent ([fig. 77] A), and ends behind at the blastopore (bl), the lips of which are continuous with the two medullary folds. As the sides of this plate bend upwards to form the closed medullary canal, the embryo elongates itself and assumes a somewhat oval form. At the same time the cranial flexure becomes apparent ([fig. 73]), and the blastopore shortly afterwards becomes shut off from the exterior. The embryo now continues to grow in length ([fig. 77] B), and the mesoblast becomes segmented. The somites are first formed in the neck, and are added successively behind in the unsegmented posterior region of the embryo. The hind end of the embryo grows out into a rounded prominence, which rapidly elongates, and becomes a well-marked tail entirely formed by the elongation of the postanal section of the body. The whole body has a very decided dorsal flexure, the ventral surface being convex. Fig. 78 represents an embryo of Bombinator in side view, with the tail commencing to project. The longitudinal section ([fig. 76]) is taken through an embryo of about the same age. In the cephalic region important changes have taken place. The cranial flexure has become more marked, but is not so conspicuous a feature in the Amphibia as in most other types, owing to the small size of the cerebral rudiment. The mid-brain is shewn at [fig. 78] a forming the termination of the long axis of the body, and the optic vesicles (a´) are seen at its sides.
Fig. 78. Lateral view of an advanced embryo of Bombinator. (After Götte.)
a. mid-brain, a´. eye; b. hind-brain; d. mandibular arch; d´. Gasserian ganglion; e. hyoid arch; e´. first branchial arch; f. seventh nerve; f´. glossopharyngeal and vagus nerve; g. auditory vesicle; i. boundary between liver and yolk-sack; k. suctorial disc; l. pericardial prominence; m. prominence formed by the pronephros.
The rudiments of the mandibular (d), hyoid (e), and first branchial (e´) arches project as folds at the side of the head, but the visceral clefts are not yet open. Rudiments of the proctodæum and stomodæum have appeared, but neither of them as yet communicates with the mesenteron. Below the hyoid arch is seen a peculiar disc (k) which is an embryonic suctorial organ, formed of a plate of thickened epiblast. There is a pair of these discs, one on each side, but only one of them is shewn in the figure. At a later period they meet each other in the middle line, though they separate again before their final atrophy. They are found in the majority of the Anura, but are absent according to Parker in the Aglossa (Pipa and Dactylethra ([fig. 83])). They are probably remnants of the same primitive organs as the suctorial disc of Lepidosteus.
Fig. 79. Transverse section through a very young tadpole of Bombinator at the level of the anterior end of the yolk-sack. (After Götte.)
a. fold of epiblast continuous with the dorsal fin; isx. neural cord; m. lateral muscle; asx. outer layer of muscle-plate; s. lateral plate of mesoblast; b. mesentery; u. fold of the peritoneal epithelium which forms the segmental duct; f. alimentary tract; f´. ventral diverticulum which becomes the liver; e. junction of yolk-cells and hypoblast-cells; d. yolk-cells.