DEVELOPEMENT OF REPTILIA. 



635 



neural arches of four vertebral segments are plainly indicated in 

 the developement of the reptilian cranium. The haemal arches 

 make their first appearance as pairs of slender rib-like bones : 

 the foremost, fig. 444, a, becomes the palato- maxillary arch ; l the 

 next early shows more clearly its division into the pleur- and 

 hrem-apophysial parts of the tympano-mandibular arch, ib. b ; the 

 third, ib. c, is the hyoid arch : the fourth has no longer the 

 cephalic relation which it shows in Fishes ; and the four neura- 

 pophyses are matched below by only three hoemapophyses in the 

 reptilian cranium. 



In the oviparous Snakes, Natrix torquata, e.g., a certain 

 progress in the developement of the embryo is found to have 

 been made when the egg is laid, and the rest is completed and 

 the vouno- extricated in the course of about two months, sooner 

 or later, according to the surrounding temperature. 



When developement has advanced to the formation of the 

 amnios about the embryo, the head is distinct, and shows the eye- 

 ball and ear-sac ; also the maxillary and mandibular processes and 

 the beginning of the hyoid, with the intervening depressions, mis- 

 called ' branchial clefts : ' the heart, as a sigmoidally bent tube, 

 fills the concavity between the frontal process and the chest: 

 the allantois has protruded, as a globular vesicle, about the size 

 of the head ; and beyond its emergence the tail forms a single 

 spiral coil : the vascular area on which the vitelline vessels ramify 

 covers half the food-yolk. The long trunk of the Serpent grows 

 in a series of decreasing spirals, and when five or six are formed, 

 the rudiment of the liver and the primordial kidneys are dis- 

 cernible. Fig. 444 shows the embryo at this period magnified 

 four times : a is the amnios, b 

 the allantois, d its tubular stem, 

 produced from the cloaca, or 

 1 urachus : ' the front view of the 

 head shows the ' frontal process,' 

 o, and the bases, n, of the palato- 

 maxillary haemal arch. The pri- 

 mordial kidneys are remarkable 

 for their length : c indicates a 

 portion of the vitellicle. With 

 the dilatation of the fore-part of 

 the alimentary canal indicating 

 the stomach, a small appendage, the pancreas, appears, 



The lungs are next seen as a 



445 



Embryo Snaka (Natrix). CCCXXX. 



the beginning of the intestine. 



marking 



1 cccxxx. taf. vii. fig. 11, e, ' Obcrkiefcr. 



