92 TEXT-BOOK OF EMBRYOLOGY. 



mesoderm outside of the embryonic disk is an intermediate step between the 

 formation of mesoderm entirely within the embryonic disk and its formation 

 around the entire vesicle, as in the hypothetical case. 



Neither in Peters' nor in Graf Spec's ovum is any embryonic body cavity 

 present. But in both cases a very large cavity exists between the mesoderm of 

 the yolk sac and that of the chorion. This cavity the extraembryonic body 

 cavity (exocodom) probably arises by a splitting of the extraembryonic meso- 

 derm into two layers, parietal and visceral, just as the embryonic body cavity in 

 other Mammals is the result of a splitting of the intraembryonic mesoderm 

 (p. 83). The splitting would occur as shown in Fig. 89, C. The parietal 

 layer which with the trophoderm becomes the chorion, then grows rapidly and 

 becomes widely separated from the visceral layer, the latter with the entoderm 

 constituting the wall of the yolk sac. Thus a stage is reached which is shown in 

 Fig. 89, C, and which corresponds with Peters' ovum (Fig. 83). The embry- 

 onic disk with its yolk sac and amniotic cavity occupies but a small space within 

 the chorionic vesicle. Consult also Fig. 106, showing the Bryce-Teacher ovum. 



The stage corresponding to Graf Spec's ovum would be produced by a fur- 

 ther splitting of the mesoderm in the roof of the amnion, so that finally the em- 

 bryonic disk and yolk sac remain attached to the chorion only by a band of 

 mesoderm, the belly stalk (Fig. 90; compare with Fig. 85). 



Even at this stage, no body cavity is present within the embryonic disk 

 (Fig. 86). When it does appear, however, it becomes continuous laterally with 

 the exoccelom (see Chap. XIV), and the parietal and visceral layers of meso- 

 derm within the embryonic body are continuous, respectively, with the parietal 

 and visceral extraembryonic mesoderm. 



References for Further Study. 



ASSHETON, R.: The Reinvestigation into the Early Stages of the Development of the 

 Rabbit. Quart. Jour, of Mic. Sci., Vol. XXXVII, 1894. 



ASSHETON, R.: The Segmentation of the Ovum of the Sheep, with Observations on the 

 Hypothesis of a Hypoblastic Origin for the Trophoblast. Quart. Jour, of Mic. Sci., Vol. 

 XLI, 1898. 



VAN BENEDEN, E.: Recherches sur les premiers stades du developpement du Murin 

 (Vespertilio murinus). Anat. Am., Bd. XVI, 1899. 



BONNET, R.: Lehrbuch der Entwicklungsgeschichte. Berlin, 1907. 



BONNET, R.: Beitrage zur Embryologie der Wiederkauer gewonnen aus Schafei. Arch. 

 /. Anat. u. Physiol., Anat. Abth., 1884, 1889. 



BONNET, R.: Beitrage zur Embryologie des Hundes. Anat. Hefte, Bd. IX, 1897; Bd. 

 XVI, 1901. 



BRYCE, T. H.,and TEACHER, J. H.: Early Development and Imbedding of the Human 

 Ovum. Glasgow, 1908. 



HARPER, E. H.: The Fertilization and Early Development of the Pigeon's Egg. Am. 

 Jour, of Anat., Vol. Ill, 1904. 



HARTMAN, C. G.: Studies in the Development of the Opossum. Jour, of Morph., Vol. 

 XXVII, 1916. 



