24 OUTLINES OF CHORDATE DEVELOPMENT 



6. The Formation of Mesoderm and Enter occels 



In the gastrula the rudiments of the middle germ layer are 

 found in a pair of longitudinal mesoderm bands of cells lying 

 either side of the chorda in the dorso-lateral regions of the 

 endodermal archenteric wall. Posteriorly these bands diverge 

 and pass either side of the blastoporal opening nearly or quite 

 to its ventral side. The extent of the mesoderm, as of the 

 neural plate and notochord, is increased chiefly by the addition 

 of cells from this blastoporal region (germ ring). At the close 

 of gastrulation therefore we can distinguish two general regions 

 of mesoderm; first, that lying either side of the anterior part 

 of the chorda, formed from the inflected dorso-lateral margins 

 of the blastopore and known as gastral mesoderm, and second, 

 that formed from the lateral and ventro-lateral margins of the 

 blastopore, known as peristomial mesoderm. The gastral 

 mesoderm is the earlier formed and remains limited to the 

 anterior region, while the peristomial mesoderm forms later 

 and during a long period after gastrulation is completed, and 

 really constitutes all the mesoderm posterior to the very 

 limited, and anterior gastral mesoderm. At the close of 

 gastrulation the only difference between the two kinds of 

 mesoderm is that of time and place of origin for they are directly 

 continuous and not visibly differentiated from one another. 



The depression of the notochordal plate, consequent upon 

 the formation of the nerve cord, results in a rather sharp 

 folding longitudinally of the dorso-lateral mesoderm bands, 

 and the formation from the archenteric cavity there of a pair 

 of longitudinal grooves (Fig. 8, B). These grooves are the 

 first indications of the enteroccelic cavities and their walls are 

 to be spoken of as the mesoderm folds. The enteroccelic 

 grooves extend nearly the entire length of the archenteron 

 and, though at first shallow, rapidly deepen, particularly in the 

 anterior region. The mesoderm folds soon become sharply 

 differentiated from the adjoining chorda and endoderm (Fig. 

 8, (7), and very early their continuity becomes interrupted by 

 the appearance of paired transverse folds dropping down from 



