FORMATION OF THE EMBRYO IN THE FOWL'S EGG. 633 



Protoi-ertebrx, Chorda Dorsalis, and Vertebral Column. On the 

 first appearance of the dorsal plates and medullary groove, at the be- 

 ginning of the second day of incubation, these structures occupy the 

 anterior half of the rudimentary embryo, or that portion which will 

 afterward become the head. Immediately behind this region, and 

 slightly in front of the primitive trace, a transverse division becomes 

 apparent on each side at a little distance from the median line. This 

 division, though visible externally as a transparent line, is really situ- 

 ated in the rnesoderm, the cells of which undergo disintegration at this 

 point along a transverse plane, thus causing a separation between its 

 anterior and posterior portions. A second line of division soon follows, 

 parallel to the first and about 0.75 millimetre behind it, including be- 

 tween the two a nearly rectangular mass of the body of the embryo. 

 Almost immediately a third line appears in advance of the first ; and 

 by this means there are formed on each side two well-defined quad- 

 rangular sections of the mesoderm. (Fig. 187.) They are the precur- 

 sors of a longitudinal chain of similar divisions, appearing successively 

 from before backward, until in the fourth day of incubation they form 

 a series of twenty-one or twenty-two pairs. From their early appear- 

 ance and their resemblance to the articulations of the future vertebral 

 column, they have received the name of the " proto vertebra." 



The protovertebra3 first formed correspond in situation with the ante- 

 rior cervical region of the embryo. The second pair, in the order of 

 formation, is placed in advance of the first ; while the third pair appear- 

 immediately behind it. (Fig. 188.) At this time the closure of the dorsal 

 plates has taken place throughout the middle portion of the head, while 

 in the anterior and posterior cephalic regions the medullary groovi- 

 still open. In the cervical region, where the proto vertebra are being 

 formed, this groove has but little depth : and farther back, near the 

 situation of the primitive trace, it is wider and shallower still. As 

 additional protovertebra become visible at the end of the series, those 

 of latest formation are always at the same distance in front of the prim- 

 itive trace. This shows that they are formed from new material sup- 

 plied by a rapid growth of the blastoderm in this situation ; each pro- 

 tovertebra taking the place of that which preceded it in the order of 

 formation, but falling behind in the linear series. In an embryo show- 

 ing seven or eight pairs of protovertebrae, as in Fig. 189, the last pah- 

 is still considerably in advance of the caudal extremity ; and the re- 

 maining pairs, belonging to the dorsal region, are still to be formed by 

 the same process. At this time the medullary canal is closed through- 

 out its cephalic portion, but is still open in the cervical region at the 

 level of the third pair of protovertebraB. From this point backward it 

 becomes gradually shallower and wider, expanding to its greatest width 

 in the caudal region, where it embraces the anterior extremity of the 

 primitive trace. When the protovertebroe have reached their full num- 

 Jber, at the forty-eighth or fiftieth hour of incubation, the caudal portion 



