70 



THE EARLY DEVELOPMENT OE MAMMALS. 



deep, narrow furrow which begins just in front of the primitive knot and ex- 

 tends 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 Fig. 26. 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 contact with the anlage of the noto- 

 chord (Fig. 26, ch), so that the mesoderm can be no longer in the median line, 

 and is consequently divided into right and left parts, as above mentioned 

 in describing the formation of the notochord. As the blastopore lies at 

 or near the side of the primitive knot, it becomes included in the medul- 

 lary plate. It may remain open while 

 the medullary plate is being transformed 

 into the nervous system, and in that 

 case may establish a connection be- 

 tween the cavity of the central nervous 

 system and that of the entoderm. Such 

 a communication is termed a neurenteric 

 canal. Fig. 64 represents a wax model 

 reconstructed from the sections of a 

 human embryo in the stage of the 

 medullary plate. It shows clearly the 

 form of the plate, the deep dorsal groove, 

 the opening of the neurenteric canal, 

 and the remnants of the primitive groove 

 behind the canal. As the development 

 progresses the medullary plate extends 

 further 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 recog- 

 nized. While the groove is being formed the medullary plate increases consid- 

 erably in thickness. These 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 further from this point we go, the less 

 advanced do we find the formation of the groove, so that at a certain stage there 



FIG. 26. CROSS-SECTION OF A HUMAN EMBRYO 

 OF 1.54 MM. 



f, Dorsal furrow, ek, Ectoderm, ct, Somatic meso- 

 derm. /, Beginning of the embryonic coelom. 

 g, Junction of the extra-embryonic somatic and 

 splanchnic mesoderm. df, Splanchnic meso- 

 derm. en, Entoderm. me, Mesoderm. ch. 

 Notochord. (After Count Spee.) 



