EM BR VOL OGY 203 



and swellings at the head -end into the Brain. Its walls are 

 entirely formed of epiblast. 



The tube of the alimentary canal and that of the general body- 

 cavity are formed in a totally different way. They are, broadly 

 speaking, the result of the junction and coalescence of the funda- 

 mental embryonic folds, the head-, tail-, and lateral folds. It is 

 obvious that the folding in of a single sheet of tissue, such as we 

 hitherto considered the blastoderm tube, can only result in the 

 production of a sac with a single cavity, and woidd not explain 

 the formation of the double tube. The blastoderm, however, soon 

 splits throughout its greater part into a double sheet, an upper and 

 a lower leaf. In the neighbourhood of the axis or future vertebral 

 column, beneath the neural tube, this cleavage is absent. In fact 

 the cleavage begins at some little distance on either side of the axis, 

 and thence spreads through the mesoblast horizontally to the 

 margin. The upper leaf or half of the mesoblast remains united 

 with the epiblast, and from its forming the body- walls, is called the 

 somatopleiire ; the lower half of the mesoblast, together with the 

 hypoblast, forms the alimentary canal and its tributary viscera, and 

 is therefore called the splanchnopleure. The space between the two 

 pleura or flaps is the general body- or pleuro-peritoneal cavity. 



This cleavage of the mesoblast into a somato- and splanchno- 

 pleure is not confined to the region of the embryo, but extends in 

 time over the whole of the yolk-sac. Hence the yolk-sac comes 

 ultimately to have an inner splanchnopleuric and an outer somato- 

 pleuric coat, and since, as we have seen above, the embryonic sac is 

 connected with the yolk-sac by a continually narrowing hollow stalk, 

 this stalk must be likewise double, consisting of a smaller inner 

 stalk within a larger and outer one. The narrow space between 

 these two investments of the yolk-sac is continuous with the pleuro- 

 peritoneal cavity. Long before hatching the inner stalk becomes 

 obliterated, so that the material of the yolk can no longer pass 

 directly into the alimentary canal (the walls of which were con- 

 tinuous with the walls of the inner stalk), but has to find its way 

 into the body of the chick by absorption through the blood-vessels, 

 which by this time have spread over the yolk-sac. The outer or 

 somatic stalk remains widely open for a long time as a thin and 

 insignificant continuation of the somatopleure. When in the last 

 days of incubation the greatly diminished yolk-sac, with its 

 splanchnic investment, is withdrawn into the rapidly enlarging 

 abdominal cavity of the embryo, the walls of the abdomen (them- 

 selves somatopleuric) close in and unite without regard to the 

 shrivelled, emptied, somatopleuric investment of the yolk-sac, which 

 is cast off as no longer of any use. The place where this has 

 happened is the outer umbilicus or navel, long visible on the middle 

 of the belly of the young bird. Remnants of the stalk between the 



