FORMATION OF THE EMBRYO. 735 



rently in consequence of an unequal rapidity of growth in the two blas- 

 todermic layers. Both layers are now extending outward, downward, 

 and inward, by the deepening of the lateral longitudinal folds ; tending 

 to approach the median line and thus shut in the abdomen and ali- 

 mentary canal. But the external layer, which is to form the walls of 

 the abdomen, grows more rapidly, and tends to inclose a larger space, 

 than the internal layer, which is to form the lining membrane of the 

 intestine. Wherever this happens, a separation takes place between 

 the two, at the expense of the intermediate layer, some of the cells of 

 which remain attached to the external and some to the internal blasto- 

 dermic Ia'er. 



VERTICAL SECTION OF A PORTION OP THE BLASTODERM OF THE FOWL'S EGG, 

 in process of separation into two laminae. 1. External blastodermic layer. 2. Internal 

 blastodermic layer. 3. Cells of the intermediate layer, partly drawn out into filamentous 

 extensions. Magnified 250 diameters. (His.) 



The cleavage or division of the blastoderm into two laminae, as above 

 described, does not take place everywhere simultaneously. It occurs 

 here and there, as the process of growth becomes more. active in par- 

 ticular spots. But the general course of its extension is from without 

 inward, or from the lateral borders of the embryo toward the median 

 line. It does not, however, reach the median line, but leaves a con- 

 siderable space around the chorda dorsalis and on each side of it still 

 undivided, when the lateral portions of the blastoderm are already 

 completely separated into their two laminse. 



By the separation of the laminse of the blastoderm thus effected, a 

 space or interval (Fig. 252, 5) is left between the two. This space, when 

 the closure of the abdominal walls is accomplished, becomes the perito- 

 neal cavity. The cells of the intermediate layer subsequently give rise 

 to the development of muscular tissue ; and that portion which, in the 

 separation of the two laminse, continues adherent to the external layer, 

 produces the voluntary muscles of the chest and abdomen. The por- 

 tion remaining adherent to the internal layer, on the other hand, pro- 

 duces the involuntary muscular coat of the alimentary canal. In Figure 

 252, II., III., and IV., these two portions of the intermediate la} ? er, 

 which give rise respectively to voluntary and involuntary muscular 

 tissue, are seen shaded with parallel lines. 



Primitive Vertebrae Formation of the Spinal Column and it* Mus- 

 cles. Already on the second day of incubation there have appeared, 

 on each side of the chorda dorsalis and medullary canal, a number 



