148 



DEVELOPMENT OF THE FROG'S EGG [Ch. XIV 



-M 



is the coelom, or body-cavity, and is at first continued into the 

 segmental jDlate. Tlie cavity in the segmental plate lies be- 

 tween the outer epithelial layer and the inner solid mass of 

 cells. 



When the medullary plate of the embryo begins to roll in 

 to form the nerve-tube, each segmental plate begins to break 

 up transversely into a series of blocks or mesodermic somites. 



The process begins first in the region 

 anterior to the middle of the embryo 

 (Fig. 43). The mesodermic somites 

 are at first somcAvhat irregular in out- 

 line. The first well-marked somite lies 

 at about the level of the ganglion of 

 the vagus nerve. In front of this there 

 are traces of another somite wliich is 

 partially broken up into loose mesen- 

 chymatous tissue. Still further for- 

 ward, the series of somites is replaced 

 by loose mesenchyme. In the frog the 

 number of head-somites (or structures 

 ■'"EoS.i^a'rrif'rSLl ™"-««P°'«l!ng to them) is uncertain. 

 MS. Mesobiastic somites. At first the primitive segments or 

 ml Si?"''''"- ''''• ""'"" «o"^ites are not separated from the 



lateral sheets of mesoderm, but almost 

 immediately after the segmental plate has begun to break 

 up transversely into somites, these begin to separate also 

 from the lateral mesoderm. This separation appears first 

 in the intersegmental borders. At this time the medullary 

 folds have met to form a closed tube. Posterior to the 

 fourth segment, the segmental plate is beginning to break 

 up into blocks, but these have, as yet, no sharply marked 

 outer or ventral boundaries. The body-cavity of the lateral 

 mesodermal sheet is at first, as we have seen, sometimes con- 

 tinued into the cavity of the segmental plate, but when the 

 constriction of the plate from the lateral sheets takes place, 

 this communication (the communicating canal) is lost. Even 

 in the younger stages there is a differentiation of a peripheral 

 epithelial layer surrounding the dense central mass or kernel 

 of the somites.* This peripheral part is represented on the 



