DEVELOPMENT. 205 



be termed the uterus), sometimes till the embryonic development 

 is completed. 



The males always possess external organs of copulation, to which 

 in the females similarly arranged rudiments (clitoris) correspond 

 In Snakes and Lizards (Plagiotrema) these organs consist of two pro- 

 trusible hollow tubes, which are either smooth or covered with spines 

 and lie retracted in a pouch-like cavity behind the cloaca. When 

 protuded their surface is traversed by a groove which conveys the 

 sperm from the genital openings of the cloaca. In the Chelonia and 

 Crocodilia, on the other hand, an erectile penis supported by fibrous 

 bodies projects on the anterior wall of the cloaca. This penis also 

 has a groove in which the semen is received and passed on, but it 

 cannot be invaginated like the two penises of Snakes and Lizards. 

 Copulation always leads to the fertilization of the ova within the 

 body of the mother. But few Reptiles, e.g., Pelias berus amongst 

 the Snakes, and the Blindworm amongst the Lizards, are viviparous. 

 Most forms are oviparous, and bury their eggs in damp earth in 

 sheltered warm spots, and take no further trouble about their fate. 

 Some of the Pythons, however, are an exception to this ; inasmuch 

 as they coil their body together over the eggs which they have laid, 

 and afford warmth and shelter to the developing brood. 



The developmental history * of the Reptiles is very similar in its 

 general features to that of Birds. The ovum is relatively large, and 

 is sometimes surrounded by a layer of albumen within the shell. 

 The segmentation is partial and leads to the formation of a discoidal 

 blastoderm, with primitive groove and medullary folds. Before the 

 medullary folds have closed, a transverse depression appears at the 

 dilated anterior end of the medullary groove ; this depression is the 

 head fold, which leads to the origin of the cranial flexure, a feature 

 always found in the higher vertebrates. [The cranial flexure is 

 found in all vertebrates except Amphioxus.] The embryo which at 

 first lies flat on the yolk, becomes gradually more and more sharply 

 marked off from the latter, for the ventral walls of the boat-shaped 

 body grow together, and leave only a small opening (umbilicus). 



* C. E. v. Baer, " Ueber Entwickelungsgeschichte der Thiere," II., Konigs- 

 berg, 1837. 



H. Rathke, " Entwickelungsgeschichte der Natter," Konigsberg, 1839. 



H. Rathke. " Ueber die Entwickelung der Schildkroten," Braunschweig, 

 1848. 



H. Rathke, " Untersuchungen iiber die Entwickelung und den Korperbau der 

 Crocodile." Braunschweig, 1866. 



L. Agassiz, "Embryology of the Turtle," Contributions to the Nat. Htet., 

 etc. II., Boston, 1857. 



