EMBRYOLOGY 383 



the neural tube is formed, it becomes cut off from the dorsal wall 

 of the archenteron as a solid rod of cells. This is the notochord or 

 chorda dorsalis, and although when first formed it does not extend 

 to the anterior end of the body it does so later. Its cells undergo a 

 considerable amount of modification, becoming highly vacuolated 

 and producing the characteristic notochordal tissue. The noto- 

 chord, then, arises from the mid-dorsal wall of the archenteron, and 

 it is looked upon as a structure of such importance that the posses- 

 sion of a supporting rod of tissue derived in this way, and exhibiting 

 its striking structural characteristics, is regarded as being one of the 

 fundamental attributes of the Phylum CHORDATA, marking it off 

 from all other Phyla. 



While the neural tube and notochord have been forming, the 

 front part of the lateral grooves has given rise to a series of hollow 

 sacs, the mesodermal somites. These arise by the grooves closing 

 off from the gut on each side in pairs of diverticula, one behind the 

 other. This is the first indication of metameric segmentation, 

 another fundamental characteristic of the Chordates. These out- 

 growths, as we have seen, are hollow from the beginning, and their 

 cavities represent the ccelom, which since it has been derived directly 

 from the gut in this particular type, is termed an enterocoel. A 

 number of such pairs of somites are formed, but with their more 

 rapid development we find that the posterior ones arise as solid 

 blocks of cells in which the cavity is formed later. The first of the 

 pouches to arise in the manner just described is really the third 

 pair of somites, there being two other pairs situated more anteriorly. 

 The first two pairs of somites arise in a somewhat similar manner, 

 but as four independent sacs growing out from the anterior end of 

 the dorsal archenteric wall in front of and not connected with the 

 lateral grooves. When the enteroccelic pouches and the notochord 

 have been cut off from the archenteron, the cavity left surrounded by 

 entoderm can be spoken of as the definitive gut the enteron or 

 mesenteron. 



Turning now to the enteroccelic pouches we see from the manner 

 of their formation that they come to constitute a group of cells 

 interposed between the ectoderm and entoderm, and so they are 

 termed the middle layer or mesoderm (or mesoblast). At the front 

 end the mesodermal somites of the two sides are opposite, but 

 further back they come to be alternate. The pouches, as soon as 

 they are formed, begin to enlarge by extending in a ventral direction 

 between the entoderm and ectoderm, and their walls differentiate 

 into three regions. The outer portion adjoining the ectoderm 

 becomes thin and is known as the parietal or somatic mesoderm. 

 The cells surrounding the gut wall likewise thin out to form the 



