892 EMBRYOLOGY. 



2. FORMATION OF THE BLASTODERM. Soon a fluid is formed at the centre 

 of the ovulum, in the midst of the mass of vitelline globules, or rather the 

 mass of young cells that take the place of the vitellus. This fluid presses 

 the globules outwards against the inner face of the vitelline membrane, and 

 the pressure increasing as it augments, these become flattened like the 

 elements of pavement epithelium, until, finally, they constitute a membrane 

 lining the zona pellucida. This is the blastoderm. 



The blastodermic vesicle (as it has been termed) does not remain 

 simple, but soon divides into two layers : an external and internal blas- 

 todermic layer, which are readily recognisable, as the elements of the first 

 are pale or slightly granular, while the cells of the second are filled with fat 

 granules, and consequently are less distinct and darker coloured. 



3. APPEARANCE OF THE EMBRYO. While the blastoderm is dividing into 

 layers, there is observed another modification taking place in the ovulum, 

 from which results the first lineaments of the embryo. As soon as tho 

 blastodermic vesicle is formed, a small round thickening takes place at a 

 point of its surface, due to a multiplication of cells. This mass destroys the 

 transparency of the blastoderm, and is designated the embryonic spot or 

 area germinativa. 



The area germinativa is the centre where commences the separation of 

 the blastoderm into external or internal layers. The middle layer, which 

 is to form the foetal organs, does not extend beyond this area. 



By a proliferation of the elements of the external and middle layers, the 

 area germinativa increases in surface and thickness ; it becomes dark, 

 prominent, and buckler-shaped, and is named the opaque area ; this is soon 

 transparent in its centre the area pellucida. The area pellucida becomes 

 elongated and constricted in its middle to form the embryonic area ; in tho 

 middle of this appears a dark line, the primitive trace or furrow, and below is 

 perceived a round cord the chorda dorsalis which develops the spinal 

 axis. 



The inner layer, in its turn, becomes two layers at the point cor- 

 responding to the embryo, and this gives rise to the middle layer. 



In fine, towards the twelfth day after fecundation, the ovum, having 

 arrived in the uterus, measures from o-r^h to ^^th of an inch in diameter, 

 and is composed of four shells contained one within the other, three of 

 which are complete, viz : 1, The zona pellucida ; 2, The external layer of 

 the blastoderm ; 3, The middle layer ; 4, The internal layer. At this 

 time the first membrane, destined to become the chorion, is already studded 

 with fine villosities. 



ARTICLE III. DEVELOPMENT OF THE BLASTODERMIC LAYERS. 



By ulterior metamorphoses, the layers of the blastoderm, give rise to the 

 annexes of the foetus and its various organs. Therefore it is that these 

 layers receive other names than those which designate their situation. 

 Thus the external, or serous, is named the sensitive lai/er, because it forms the 

 skin and organs of sense ; the middle is the vascular or germinative, as it 

 contains the principal vessels of the embryo, and in its substance are 

 developed the organs of locomotion ; lastly, the internal layer is termed the 

 mucous or intestino- glandular, in consequence of its constituting the mucous 

 membranes, and its chief portion forming the intestines and their glands. 



Each blastodermic layer, then, in becoming developed, furnishes the 

 intrafoetal and extrafcetal parts. We will study this development, having 



