68 



THE EARLY DEVELOPMENT OF MAMMALS. 



A. p. A.o. 



md.F 



in front, also behind, where, however, it gradually fades out. At the same time 

 that the medullary plate is being thus differentiated, the central portionbe comes de- 

 pressed, making the conspicuous furrow, md.F, which begins just in front of the primi- 

 tive knot and extends nearly to the anterior edge of the medullary plate. This 

 axial depression is known as the dorsal furrow. Its appearance is shown in cross- 

 section as illustrated by figure 36, /. The furrow is narrow and deep. Its upper 

 edge is rounded or curving. By the formation of the furrow the ectoderm of the 



medullary plate is brought into actual con- 

 tact with the anlage of the notochord (Fig. 

 36, ch), so that the mesoderm can be no 

 longer in the median line and is conse- 

 quently divided into right and left parts, as 

 above mentioned in describing the formation 

 of the notochord. As the blastopore lies at 

 or near the primitive knot, it becomes 

 partly included in the medullary plate. It 

 may remain open while the medullary plate 

 is being transformed into the nervous system,' 

 and in that case may establish a connection 

 between the cavity of the central nervous 

 system and that of the entoderm. Such a 

 communication is termed a neurenteric canal. 

 Figure 79 represents a wax model recon- 

 structed from the sections of a human em- 

 bryo in the stage of the medullary plate. 



Md 



Kn 



pr.s 





FIG. 35. SURFACE VIEW OF THE EMBRYONIC 



SHIELD OF A DOG EMBRYO, WITH MEDULLARY It: sn OWS clearly the form of the plate, the 



PLATE. 



deep dorsal groove, the opening of the 

 A.o, Area opaca. A.p, Area pellucida. Kn, Hen- neurenteric canal, and the remnants of the 



sen's knot. Md, Medullary plate. md.F, ... . 



Medullary furrow, pr.s, Primitive streak, x .P rimitlve groove behind the canal. As the 

 15 diams. development progresses the medullary plate 



extends farther backward and encroaches 

 upon the territory of the primitive streak until this latter is obliterated. 



The Medullary Groove. Almost or quite as soon as the medullary plate is 

 formed its lateral portions begin to arise on each side, so that the two halves of 

 the plate together form a broad open trough known as the medullary groove, 

 into which, of course, the dorsal groove is merged, so that it no longer can be 

 recognized (compare Figs. 22 and 147). While the groove is being formed 

 the medullary plate increases considerably in thickness. The nuclei multiply 

 rapidly and lie irregularly scattered at various heights. The ectoderm alongside the 

 medullary plate or groove thins out still further. The development is most rapid 

 at a point corresponding to the posterior region of the future head. The farther 

 from this point we go, the less advanced do we find the formation of the groove. 



