REPORT ON THE GREEN TURTLE 15 



Another section (fig. 3) is tlirongh tlie oral involution, and shows a slice through 

 the lower part of the most solid lobe of the pituitary body {py.), the trabeculee {tr.) are 

 cut through on each side ; the curious foldings of the lips of the involution are seen 

 here as in fig. 4. 



A section on a still lower plane (fig. 5) shows well the junction of the parts that 

 form the pituitary floor, or sella turcica ; the trabeculse, iutertrabecula, and fore 

 part of the investing mass {tr.,i.tr.,iv.). Here the lower part of two of the pituitary 

 lobes are cut through, and the sixth nerve (6) is seen passing outwards, having been 

 severed from the floor of the hind-brain, towards the external rectus muscle ; out- 

 side this is seen a fragment of the trigeminal nerve (5^'^). The internal carotids (i.e.) 

 appear further forwards. 



These embryos of so large a reptile, although only a third the size of the Tadpole 

 of a Common Frog, show many most important things. Among these are the various 

 visceral folds and clefts ; the large size and great length of the intertrabecular bar, 

 the size and height of the post-pituitary wall, with the end of the notochord ; also 

 the commencement of the pituitary body, of the infundibulum, and of the olfactory tract, 

 whilst the optic nerve is still hollow. 



The olfactory nerves are still quite solid and independent of the hollow olfactory 

 tracts ; beneath them, and beneath the eyeballs also, a curious gap or cleft is seen on each 

 side, which at once suggests the idea of a series of clefts, as there are also a series 

 of folds truly homologous with the clefts and folds behind the mouth ; and, indeed, the 

 mouth itself, with its opercular fold, the hind part of the upper lip, may be considered to 

 belong to the same category. 



Much earlier embryos would have been recpired to show the origin of the third 

 pair of cranial nerves from the dorsum of the middle cerebral vesicle. I am satisfied 

 that the sixth pair (abducentes) grow from the inferior surface of the hind-brain. 



As the embryonic cartilage has become almost hyaline, in most tracts where that 

 tissue has to be developed, a very short time suffices to bring this stage on to the 

 next, in which almost all the endo-skeletal structures will be well marked out and laid 

 ilown in that solid tissue. 



Fifth Stage. Embryos one inch and a third in length, measured along the curve. — 

 («.) Lateral vieiu of embryo. — In actual size these embryos equal a honey-bee, yet their 

 development is very perfect ; and as the chondro-skeleton is now complete they are at a 

 stage which represents the permanent skeleton of cartilaginous fishes (Marsipobranchii 

 and Elasmobranchii), and the temporary skeleton of amphibian larvae. 



Small as these embryos are, they have assumed already much of the form which is 

 permanent in this gigantic species. The chick of the Common Fowl arrives at this 

 stage by the end of the first week of incubation, but is hatched in a much more im- 



