66 MANUAL OF ANIMAL BIOLOGY 



by a sperm, begins to divide by mitotic cell division, the process 

 being known as cleavage. As a result of cleavage, two cells, then 

 four, eight, sixteen, etc., are formed. These cells remain attached 

 to each other and, in Hydra, in a short time, when about 128 cells 

 are present, it is found that they are arranged in the form of a 

 hollow sphere, the wall of which is composed of a mosaic of cells 

 closely fitted together. This stage in the development of Hydra 

 or other forms is known as a blastula, and it is broadly compa- 

 rable with the permanent structure of Volvox, previously studied. 

 (W. fs. 30, 31.) 



In the development of Hydra the blastula is only a transient 

 condition. The ectoderm cells, of which the wall of the blastula is 

 composed, continue to divide and soon a special group of very 

 active cells arises at one pole of the sphere. The latter are some- 

 what differentiated from the outer ectoderm cells, and they are 

 destined to form the inner layer, endoderm, which, as will be 

 remembered, lines the enteric cavity and is responsible for the 

 nutrition of the animal. The young Hydra, in this two-layered 

 condition, with outer ectoderm and inner endoderm, is known as a 

 gastrula. After gastrulation the embryo forms a thin inner mem- 

 brane and a thick, outer chitinous shell — both of which are prod- 

 ucts of the ectoderm cells — becomes free from the parent and 

 falls to the bottom of the pond. Later the embryo emerges from 

 the shell, develops tentacles, and the ectoderm and endoderm cells 

 become specialized as in the mature Hydra. Thus Hydra remains 

 essentially as a permanent gastrula made up of the two cellular 

 layers, ectoderm and endoderm, separated by the non-cellular 

 mesogloea. 



Because of the fact that the body of Hydra and related forms is 

 composed of two cellular layers, the ectoderm and endoderm, it is 

 known as a diploblastic animal in contrast with the higher forms 

 of triploblastic animals, in which the body with the various 

 organs arises from three original cellular layers, ectoderm, endo- 

 derm, and mesoderm. In the triploblastic animals the very impor- 

 tant and highly specialized layer of mesoderm lies between the 

 ectoderm and endoderm. The ectoderm and endoderm of Hydra, 

 together with the mesoderm of the triploblastic animals, are called 

 the primary germ layers. In all forms of animals they arise very 

 early in development by a differentiation of the embryonic cells, 

 and it is by continued differentiation of the cells of these primary 



