FROM SIMPLE CELL TO COMPLEX ANIMAL 39 



54. Blastula and Morula. As cleavage continues the blasto- 

 meres remain associated in a spherical mass. The individual 

 cells project beyond the general surface not unlike the lobes 

 of a mulberry, and for this reason this stage is called the morula 

 or mulberry stage (Fig. 13, 2). By the growth of the cells and 

 by the imbibition of water the morula may become a hollow sphere 

 of cells (blastula) the central cavity of which is filled with fluid. 

 The cavity is termed the segmentation cavity (Fig. 13, s.c.). 

 Some animals, such as Volvox, never develop beyond the blastula 

 stage. 



55. Gastrula. In those eggs in which the segmentation is 

 total, a next important step is the pushing in of that side of 

 the blastula which corresponds to the original nutritive pole. 

 The process is known as imagination, and the product as a 

 gastrula (Fig. 13, 4). It takes place much as one might suppose 

 one side of a hollow rubber ball to be dimpled or infolded by the 

 exhaustion of the air within. The gastrula is to be described 

 as made up essentially of two layers of cells, one external and 

 called ectoderm, and one within called entoderm (Fig. 13, 4). 

 The segmentation cavity may be wholly obliterated; in that case 

 the entoderm and ectoderm come to lie in contact. The cavity 

 of the invagination of the gastrula is the archenteron or embryonic 

 digestive tract; the opening into it, that is, the mouth of the 

 gastrula, is the blastopore (Fig. 13, bp). Such an organism as 

 Hydra (see Fig. 81) may be looked upon as a permanent gastrula, 

 somewhat modified in form. In morulas in which the seg- 

 mentation cavity is small and the cells at the nutritive pole are 

 large (Fig. 13, C, 4) this simple condition is much obscured, 

 and invagination as described above becomes impossible. Nev- 

 ertheless early in development the cells which produce the two 

 primitive layers are to be distinguished, and their relations are 

 always substantially as detailed. If the term gastrula is applied 

 to these we have to say that they are formed in some other way 

 than by ordinary invagination. 



56. Library Reference. Let students report briefly on gastrulation by over- 

 growth (epibole), and by delamination. Compare the results attained by the 

 various methods. Note what is constant in the methods and in the results. 



