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CHAPTER XLI. 



ON THE DEVELOPMENT OF THE EMBRYO. EARLY CHANGES IN THE 



BIRD'S OVUM. AREA PELLUCIDA, AREA GERMINITIVA, AREA VAS- 

 CULOSA. SEROUS AND MUCOUS LAMINAE. INVESTING MEMBRANE 

 AND MEMBRANA INTERMEDIA OF REICHERT. CHANGES IN 



THE MAMMALIAN OVUM. PRIMITIVE STREAK. FORMATION OF 



DORSAL AND VENTRAL LAMINAE. FORMATION OF THE AMNION. 



BRANCHIAL FISSURES AND ARCHES. HEART AND LARGE ARTE- 

 RIES. ALLANTOIS. DEVELOPMENT OF THE HUMAN EMBRYO. 



IN the last chapter, we described the earliest changes resulting 

 from impregnation, and traced them up to the formation of the 

 germinal membrane, and the first appearance of the embryo. We 

 have now to consider the manner in which the development of the 

 embryo takes place, and the mode in which the different organs 

 are evolved. It will be most convenient to describe, as briefly as 

 possible, the earliest changes occurring in the bird's egg, in the 

 first instance, and then to consider the development of the mam- 

 malian ovum, and that of the human subject. 



Early Changes in the Bird's Ovum. In considering the nature 

 of the earliest changes in the egg, resulting from impregnation, but 

 which are dependent for their commencement and continuance 

 upon a temperature varying from 95 to 104 F., we must premise 

 that the yolk of the bird's egg consists of two distinct portions, 

 one which undergoes segmentation, and alone takes part in the 

 formation of the embryo the germ-yolk ; the other, and by far the 

 larger portion, does not undergo segmentation, and takes no part 

 in the formation of the germ, but provides the pabulum for its 

 development and nutrition, whence it is termed the food-yolk. 



Area Pellucida, Area Germinitiva, and Area Vasculosa. A few 

 hours after the egg has been exposed to a hatching heat, it will be 

 found that the germ has become less adherent to the vitelline mem- 

 brane. It has assumed a more membranous appearance ; and about 

 the sixth or seventh hour, a clear space may be discerned in its 



