932 OF GENERATION EMBRYONIC DEVELOPMENT. 



soblastic, and an internal or hypoblastic layer is seen as in the egg of the 

 fowl, and the process of development is in all essential respects in its early 

 stages identical with that of the egg of the Bird. From the epiblast the 

 brain and spinal cord, the special modifications of the nerves ministering to 

 special sense, as the Retina, Auditory hairs, etc., and the epidermis with its 

 epithelial prolongations into the mouth, salivary ducts, and cloaca, are de- 

 rived. The mesoblast gives origin to the muscles, bones, connective tissue, 

 and dermis, the cerebro-spinal and sympathetic nerves, the genito-urinary, 

 and the chylopoietic system, and, lastly, from the hypoblast arise the epithe- 

 lium of the digestive canal and of the ducts of the glands connected with it 

 (except the salivary), as well as the cells constituting the parenchyma of the 

 liver and pancreas, and the epithelium of the respiratory apparatus. Whilst 

 this is progressing the fine membrane covering and inclosing the blastoder- 

 mic vesicle begins to show small mammillary eminences, whence the name 

 which has been suggested for this membrane, of Chorion primitivum or Mem- 

 brana ovi externa. The ovum at this period, therefore, consists of the exter- 

 nal villous membrane or primitive chorion ; of the blastodermic vesicle, 

 composed of two layers ; and of the cellular yolk. As in the Bird, the area 

 germinativa at its first appearance has a rounded form, but it soon loses 

 this, first becoming oval and then pear-shaped (Plate II, Fig. 11). The 

 centre becomes clear, and constitutes the area pelluclda, and this is bounded 

 by a more opaque circle, the area opaca. The first appearance of the embryo 

 in the form of the primitive trace with the subsequent elevation of the 

 lamina dorsales, their junction in the middle line, and expansion anteriorly 

 into three vesicles (Plate II, Fig. 126), afterwards to be developed into the 

 prosencephalon, mesencephalon, and epencephalon ; the formation of the amniou 

 by the growth of two folds of the epiblast, and the growth of the allantois 

 into its cavity differ in no essential respect from the similar processes in the 

 chick, though the allautois never attains the same degree of development iu 



FIG. 337. 



At, '/ 



It, 



/> trutJi, tlU -Jy t eVl 



Transverse section through the Embryo of the Chick at the close of the first day of incubation, mag- 

 . nified about 100 diameters: ch, chorda dorsalis; A, external serous or corneal layer; m, medullary por- 

 tion of serous layer ; Pv, primitive groove between the dorsal lamina? Rf and HI ; d<l, intest itial ejiithe- 

 lial or glandular layer (mucous layer) ; uwp, prevertebral mass, in which the primary or protovcrtehra; 

 are formed, and which ia continuous with the middle lamina, sp ; uwh, fissure in the middle lamina, 

 presenting the first indication of the pleuro-peritoneal cavity, and of the subsequent divi.sion of the 

 middle lamina into two layers. 



the mammal. A microscopic examination of the Amniotic membrane in 

 the Human subject shows that it consists of an inner layer of tessellated epi- 

 thelium, and an outer layer, which even at the fourth week presents spindle- 

 formed corpuscles, and at the seventh week has become well-marked con- 

 nective tissue. In some animals it is provided with muscular fibres, and 

 performs distinct movements; but it never in any species possesses proper 

 vessels. From its mode of formation, the Amnion is directly continuous 

 with the skin of the embryo, and the Amniotic liquor in which the foetus 

 floats occupies the space between the inner layer and the skin. 



777. As the development of the embryo progresses, the walls of the ab- 

 domen and the coats of the intestines begin to be formed by the bending 



