IV 



COELENTEKATA 



55 



word, which literally means "mulberry," is used to characterize a 

 condition where the cells which constitute the embryo form a com- 

 pact spherical mass. The morula stage is reached by the proliferation 

 of cells from the walls of the blastula in sufficient number to fill up 

 the interior. Whether these cells are budded from all parts of the 

 blastula wall or only from a certain area of it, has not been made out. 

 In the case of the eggs of the free-swimming medusae, however, it 

 is beyond all question that these cells are budded only from one end 

 of the blastula, and none of Brauer's figures are inconsistent with 



FIG. 28. Two methods of 

 formation of the blastula 

 in Tubularia mesem- 

 bryanthemum. (After 

 Brauer. ) 



A, egg segmenting in normal 

 method. B, egg segmenting 

 in abnormal method. 



COS 



FIG. 29. Formation of endoderm in Tubularia 

 mesembryanthemum. (After Brauer.) 



A, budding of endoderm cells from blastula wall. 

 B, morula stage, eoe, spaces which will ultimately 

 form the coelenteron ; ect, ectoderm ; end, endoderm ; 

 i-nst, interstitial cells. 



the assumption that this is the case with the blastulae of Tubularia 

 also. In any case a solid morula stage is soon reached in which the 

 whole interior of the blastula becomes clogged up with a mass of cells. 

 This mass of cells constitutes the rudiment of the endoderm of the 

 adult, whilst the original blastula wall forms the ectoderm. In the 

 solid mass of endoderm spaces begin to appear owing to the absorp- 

 tion of some of the central cells. These spaces (coe, Fig. 29) 

 eventually coalesce so as to form one cavity which is the gastral 

 cavity or coelenteron of the adult. The embryo has now the form of 

 a circular disc, and from its edges a series of blunt protuberances 

 grow out. These are the rudiments of the aboral tentacles of the 

 adult : they are bent towards the future aboral end of the body. At 



