734 FORMATION OF THE EMBRYO. 



the head and another portion the tail ; and consequently that every one 

 of the future organs of the embryo has its point of origin already fixed. 



Division of the Blastodermic Layers. The blastoderm when first 

 formed consists, as above described, of two layers of cells ; those of the 

 external layer being cylindrical and compact, those of the internal, 

 larger, rounded, and more loosely connected. The outer blastodermic 

 layer forms the tegumentary surface of the body and the cavity of the 

 cerebro-spinal axis ; the inner is converted into the mucous membrane 

 of the alimentary canal. But between the two there soon appears 

 another formation of cells, which is sometimes spoken of as the third 

 or " intermediate" blastodermic layer. The cells of this layer are in 

 immediate contact with, and more or less adherent to, those of the two 

 others ; but they are rounded in form and rather loosely united, in com- 

 parison both with those above and below. The intermediate layer, in 

 the blastoderm of the fowl's egg, is distinctly formed, according to 

 Foster and Balfour, in the first twelve hours of incubation. 



The exact number and designation of the fundamental layers of the 

 blastoderm has been and still is the main point of discrepancy in the 

 writings of embryologists. There is no difference of opinion as to the 

 existence or destination of the two principal layers, namely, the external 

 and internal, which are the first to make their appearance, as above 

 described. They form respective^ thQ basis for the production of the 

 external sensitive integument and cerebro-spinal axis on the one hand, 

 and for the lining of the alimentary canal with its adjacent glandular 

 organs on the other. But the intermediate portion, formerly described 

 as the u vascular layer," is connected both with the organs of animal 

 life and with those of digestion and nutrition. It is, therefore, by some 

 regarded as an independent layer, equal in original importance to the 

 other two ; by others as an accessory formation, destined to aid in the 

 development of both the external and internal parts. According to 

 His, 1 whose observations are among the most extensive and valuable 

 in the department of embryology, the most appropriate enumeration is 

 the older one, of an external and internal blastodermic layer ; since the 

 cells of the intermediate portion remain attached partty to the outer 

 and partly to the inner layer, when the separation between the two 

 takes place in the manner now to be described. 



Immediately underneath the medullary canal, along the axial line of 

 the body of the embryo, there is formed in the intermediate layer of 

 the blastoderm a cylindrical cord, termed the "chorda dorsalis" (Fig. 

 252, 6), which marks the situation of the future spinal column. For 

 a certain distance on each side of the chorda dorsalis, the component 

 parts of the blastoderm remain in contact with each other throughout 

 its thickness. But farther outward, toward the edges of the embryo, 

 it separates, by a horizontal division or cleavage, into two laminae, an 

 outer and inner, or upper and lower. This cleavage takes place appa- 



1 Unsere Korperform. Leipzig, 1875, p. 38. 



