DEVELOPEMENT OF BATRACHIA. 619 



The Crocodilians, like the Chelonians, are all oviparous, and 

 the process of oviposition is very similar. The eggs, of an ellip- 

 tical form and with a firm calcareous shell, are buried on the 

 shore, and left to hatch by extraneous heat. 



119. Developement of Batrachia. After impregnation of the 

 batrachian ovum the dark or germinal part of the yolk is always 

 uppermost, and its central point may be defined as the germinal 

 pole. Here begins, usually about three hours after impregnation 

 in the Frog, fig. 452, , the process of segmentation, 1 by a fissure 

 which passes in a determinate direction through the canal of the 

 yolk, dividing it into two ellipsoid masses, ib. b. About the fifth 

 hour a second cleft appears, near the point where the first com- 

 menced, crossing the first at right angles. If an ovum in this state 

 be frozen, it splits into four segments of a sphere. Fissures next 

 appear, which, in relation to the two foregoing, might be termed 

 6 equatorial,' but with varieties exemplified in e,f, g, fig. 452. New 

 ( meridional ' furrows follow, ib. h, crossed again by other ( equa- 

 torial ' ones, until the surface of the yolk presents the form of a 

 blackberry. Further subdivision proceeds to such an extent as to 

 render the surface again apparently smooth. This series of pheno- 

 mena, resulting in the formation of the germ-mass, occupies about 

 twenty-four hours, or less, according to the temperature. The 

 fissures at their first appearance show minute lines at right angles, 

 indicative of the molecular movements causing them. After the 

 surface of the yolk has resumed its smoothness on the completion 

 of the germ-mass, peripheral cells become filled with dark pigment, 

 and constitute a general ' cambium' or outer investment, fig;. 428. a. 



O J O ' 



At the point where the formation of this investment, as well as of 

 the germ-mass, began, an eminence appears by the developement 

 of new cells beneath the investment, which loses its colour at this 

 part, indicating the first rudiment of the embryo as an oval clear 

 spot, divided at its hinder end by a crescentic fissure from the 

 contiguous yolk, and with its anterior end sunk therein. The 

 embryonal cells, as they accumulate, assume a polyhedral figure, 

 and their different strata are seen by transverse sections. The 

 first superficial appearance of the embryo is an oblong rising, 



1 The phenomenon of the division and subdivision of the yolk in animals was first 

 observed by Prevost and Dumas in the ovum of the Frog (Annales des Sciences 

 Nat. t. ii.., May 1824, p. 112). Franz Bauer, in the same year, delineated partially 

 the same important phenomenon, in the beautiful drawings which he prepared for 

 Sir Ev. Home (cccxvi. pis. v. and vi.); but his employer had no appreciation or 

 comprehension of what was thus shown him. Bergmann detected, in 1841, the 

 hyaline nucleus in the centre of each subdivision of the yolk ; and the combination 

 of the spermatised cell progeny of the germinal vesicle with other elements of the 

 yolk- substance appears to be a necessary prelude to segmentation. 



