924 OF GENERATION EMBRYONIC DEVELOPMENT. 



blood, though not receiving that blood as such into its own organism ; and it 

 is through the same medium that the aeration of its own blood is effected, 

 its pulmonary apparatus being as yet inoperative. Its circulating system, 

 arranged in accordance with these requirements, presents many peculiarities 

 which mark its foetal character ; and the alteration in the course of the 

 blood, which takes place as soon as the respiratory organs come into play, 

 constitutes the essential difference between intra-uterine and extra-uterine 

 life. If, as sometimes happens, the lungs of the newborn infant expand but 

 imperfectly or scarcely at all, the circulation continues to be carried on in a 

 greater or less degree, upon its intra-uterine plan ; and this, when the pla- 

 centa is no longer capable of supplying the needed aeration, is incompatible 

 with the persistence of life. 



775. The early stages of the development of the Embryo in Vertebrata 

 are much better known in the Bird than the Mammal, on account of the 

 facilities that the eggs of these animals offer for examination, and it will be 

 expedient, therefore, to describe the course of events in them : noting the 

 principal differences that have been observed in the process as it occurs in 

 the higher class. 1 A newly-laid hen's egg is composed of a shell (Fig. 324, 

 s\ composed of an organic basis impregnated with calcic salts, and perfo- 

 rated by vertical canals, through which an interchange of gases can take 

 place. The shell is lined by two layers of membrane (Fig. 324, sm, uin}, 

 which early separate from each other at the broad end to form the air- 

 chamber. Beneath the shell-membrane is the white of the egg, or albumen 

 (Fig. 324, tv); and stretching from near the two extremities of the egg to 

 the opposite surfaces of the yolk, are two twisted portions of rather firmer 

 albumen, termed the chalazte (ch, e), which perhaps serve to keep the yolk 

 in position. The yolk is inclosed in the vitelline membrane (vt), and by far 

 the largest portion of it is composed of yellow yolk (yy) which consists of 

 spheres, never containing a nucleus, but filled with minute, highly retractile 

 granules. The yellow yolk is surrounded by a thin layer of a lighter-colored 

 yolk, the white yolk (wy\ which, dipping into it at one point, forms a flask- 

 shaped mass in its interior. The white yolk also forms a series of concentric 

 layers (halones] in the substance of the yellow yolk. It is composed of 

 spherules, for the most part smaller than those of the yellow yolk, with a 

 highly refractive nucleus-like body in the interior of each, and of larger 



1 In the following account, Foster and Balfour's Elements of Embryology has 

 been taken as a guide, and to this excellent work the student is referred for details 

 that considerations of space preclude from being here inserted ; but other work? and 

 researches which may be studied are those of Rathke, On the Development of the 

 Snake, 1839, and of the Tortoise, 1848; of Kollikcr (Miiller's Archiv, 1843, p. 68) 

 and Bagge (!)e Evolut. Strongyli et Ascarid , Diss. Inaug., 1841) on the ova of 

 Eiilozna Kolliker's Entwickeliingsgeschichte des Menschen, 1801; those of v. B-ier, 

 On the Development of the Fish, 1835; those of Mr. Newport (Philos Transact., 

 1851) and Duges (Rechercbes sur les Batraciens, 1835) on the ova of Bn1i-<n-liht ; 

 those of Bischoff (EntwickelungsgeschichtG des Hundes-eies, 1845) on the ova of the 

 Bifc/i ; those of Rernak on the Vertebrata (Untersuch. iiber die Entwickel. dec \Vir- 

 belthiere, Berlin, 1855); of Re.ichert on the Guinea-pig (Bcitriige xur Entwicke- 

 Lungsgeschiuhte des Meerschweinchens), Monatsbericht d. Akad. Berlin, 18tiO; of 

 Leurkart, contained in Wagner's Handworterbuch der Physiologic, art. ZeuLjung ; 

 of Allen Thomson, in the art. Ovum, in the supplementary volume to Todd's Cyclop. 

 of Anatomy and Physiology, 1859; of Huxley, in the Croonian Lecture for IS.'iS, 

 and in his Lectures on the Elements of Comparative Anatomy, 1864 ; of Coste ( His- 

 toirc Gener. ct Partic. du Developpement des Corps Organises, 18-17-1859) ; of Pan- 

 der, Beilra'ge zur Entwick. d. Huhnchens ; of His, Untersuchungen iiber die ersto 

 Anlage des Wirbelthierlfibes, Leipzig, 18(i8; of v. Beneden, Reeherches sur la com- 

 position et la signification de l'(Euf, 1870; of Oellacher, SehulUe's Archiv f. mic. 

 Anat., Bd. viii, 1872; Goethe, On the Development of the Bomhinator igneiis, 1875, 

 and of Parker's various papers, On the Development of the Skull, in Philosophical 

 Transactions. 



