Elementary text-book of zoology. Special part: Mollusca to man / by C. Claus ; translated and edited by Adam Sedgwick ; with the assistance of F.G. Heathcote.
- Carl Friedrich Wilhelm Claus
- Date:
- 1897
Licence: Public Domain Mark
Credit: Elementary text-book of zoology. Special part: Mollusca to man / by C. Claus ; translated and edited by Adam Sedgwick ; with the assistance of F.G. Heathcote. Source: Wellcome Collection.
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![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 Ghelonict 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 Blind worm 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 discoidai 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. y. Baer, “Ueber Entwickelungsgeschichte der Thiere,” II., Königs- berg, 1837. H. Rathke, “ Entwickelungsgescbichte der Natter,” Königsberg, 1839. H. Rathke, “ Ueber die Entwickelung der Schildkröten,” Braunschweig, 1848. H. Rathke, “ Untersuchungen über die Entwickelung und den Körperbau der Crocodile,” Braunschweig, 1866. L. Agassiz, “Embryology of the Turtle,” Contributions to the Nat. Hist., etc. II., Boston, 1857.](https://iiif.wellcomecollection.org/image/b2813378x_0207.jp2/full/800%2C/0/default.jpg)
