THE EVIDENCE FROM DEVELOPMENT. 173 



waterways, on the banks of which the inhabitants live, is thus seen to 

 be fully justifiable. The development of such an organism takes place 

 through the production of eggs (/y), which are developed in the tissues of 

 the parent sponge, and which are merely specialised portions or cells of 

 the inner layer of the parent body. The sponge-egg (Fig. 87,1), it must 

 be remarked, presents the essential elements seen in the eggs or germs 

 of all animals. It is a little speck of protoplasm, imbedded in which 



FIG.' 87. DEVELOPMENT OF A SPONGE (Olyntkus). 



we find a smaller body known as the germinal vesicle (a), and this latter 

 in turn contains a still more minute particle, the germinal spot (b). 



When such an egg is about to undergo development, the first 

 changes which occur in its substance are those collectively named 

 " segmentation." The egg is then seen to undergo a process of divi- 

 sion (2). It divides internally and successively into two, four, eight, 

 sixteen, &c., cells or divisions ; these portions ultimately becoming so 

 numerous, that the egg at the close of its segmentation, from its 

 resemblance to a mulberry, has been named a morula (3). Soon the 

 outer cells become elongated and provided with cilia (4), and by 

 means of these filaments the young organism swims freely about in 

 the water. In this stage it is known as the planula (4). Next in 

 order a central cavity and then a mouth (5, m) are formed, this aperture 

 leading into the cavity (e) of the cup. It is now named the gastrula\ 

 and its body is seen to consist of two typical layers, an outer or 

 ectoderm (c) and an inner or endoderm (d). These two germ layers, 

 as we shall hereafter note, are common to all animals in the course of 

 their development indeed, the formation of the embryo takes place 



