66 



THE EARLY DEVELOPMENT OF MAMMALS. 



less advanced, while part of the primitive streak still presents to us, more or less 

 clearly, its original condition. 



The Notochordal Canal. 



In regard to this canal our knowledge is imperfect. Any account of it 

 which we can give may need correction. It is a very small canal which runs 

 through the center of the primitive axis. It ends blindly in front, but opens 

 through the ectoderm at its posterior end, at a point corresponding perhaps 

 exactly to the position of the primitive knot. The first indication of the forma- 

 tion of the canal is an alteration in the form of the cells in the center of the primi- 

 tive axis. These cells elongate in directions at 

 right angles to the axis. Their nuclei become oval 

 and are radially placed. The change begins poste- 

 riorly and progresses forward. The radial cells 

 move apart, so that there arises a longitudinal 

 canal. It may happen -that in mammals, as in 

 birds, the canal is not actually open at its posterior 

 end. If that should be found to be the case in any 

 instance, it would not alter our interpretation, for 

 we should then consider that the walls had simply 

 closed together. There are many instances of tub- 

 ular structures being temporarily solid in embryonic 

 stages. Such a condition, for example, has been 

 observed in the oesophagus of elasmobranchs, in 

 the large intestine of birds, and in other cases. 



The opening of the notochordal canal is termed 

 the blastopore, and is supposed to be identical with 

 the blastopore of the anamniota. 



After the notochordal canal is formed the 

 blastodermic vesicle has, of course, two cavities: 

 first, the small cavity of the canal; second, the 



large main cavity of the vesicle which is surrounded by entoderm. This larger 

 space is designated as the yolk-cavity. After the canal has acquired a not in- 

 considerable length its lower wall develops a series of irregular openings (Fig. 

 22, nch) on its ventral side, by which it comes into communication with the 

 large underlying yolk-cavity. These openings grow until the ventral wall of 

 the notochordal canal is entirely lost. We then have the two cavities com- 

 pletely fused making a single cavity, bounded by a continuous layer of cells, 

 the majority of which represents the lining of the yolk-cavity, but the small 



FIG. 22. GERMINAL AREA OK A 

 GUINEA-PIG AT THIRTEEN DAYS 

 AND TWENTY HOURS, SEEN 

 FROM THE UNDER (ENTODER- 

 MAL) SIDE. 



a.o, Areaopaca. a.p, Area pellucida. 

 nek, Notochordal canal with 

 several irregular openings through 

 the entoderm. 



