630 REPRODUCTION. 



This is evident from the manner in which the local activity of nutrition 

 gives rise to the appearance of folds, running in definite directions, 

 and determining in this way the future location of the head, the tail, 

 and the sides of the body. But it is manifested still more remarkably 

 in the position of the entire embryo. The yolk of the fowl's egg has 

 a nearly regular spherical form ; and the cicatricula, as well as the blas- 

 toderm into which it is converted, is a circular spot upon its surface. 

 The ovoid form presented by the whole egg, with one round and one 

 pointed extremity, results from the deposit of albumen around the yolk, 

 in the middle and lower parts of the oviduct, after fecundation has 

 taken place. But when the rudimentary embryo first becomes per- 

 ceptible in the area pellucida, it is so placed in the large majority of 

 instances as to lie crosswise to the long axis of the egg, with its left 

 side toward the round end and its right side toward the pointed end. 

 Even before incubation has commenced, one particular portion of the 



r d 



TRANSVERSE SECTION OF EMBRYO CHICK, second day of incubation, through open portion of me- 

 dullary groove. Mg. Medullary groove. Dp. Dorsal plates. Ch. Chorda dorsalis. Ect. Ectoderm. 

 Md. Mesoderm. Enl. Entoderm. Magnified 83 times. (Kolliker.) 



circular blastoderm is destined to become the head and another portion 

 the tail ; and consequently every one of the future organs of the embryo 

 has its point of origin already fixed. 



Dorsal Plates, Medullary Canal, and Cerebro-Spinal Axis. During 

 the first day of incubation the primitive trace is bordered on each side 

 and around its two extremities by a thickened extension of the blasto- 

 derm, which rapidly assumes an elongated form and grows more rapidly 

 in the anterior than in the posterior direction. Early in the second 

 day there appear, within the embryonic spot, in front of the primitive 

 trace, two parallel longitudinal folds of the ectoderm, which project 

 above the surface, leaving between them, along the median line, a cor- 

 responding longitudinal depression (Fig. 184). These ectodermic folds 

 are known as the " dorsal plates," and the depression between them is 

 the "medullary groove." As the dorsal plates increase in height, their 

 edges curve inward, and the intervening groove, which is lined with 

 the cells of the ectoderm, becomes deeper and more capacious. By a 

 continuance of this process, the edges of the dorsal plates are more 



