40 THE PROGRESSIVE DEVELOPMENT 



relation to the organization of the egg. The concentric, spiral 

 layers (a, a 1 ) of the white are the result of several successive 

 deposits in the tube (oviduct) through which the egg passes, 

 with a boring motion, in order to reach the outer world. 



Now, as is well known, there are animals which in a full- 

 grown condition are so lowly organized as to correspond in this 

 respect to the early embryonic stages of some of ,the higher 

 animals, for instance, a fish in the degree of development of 

 its various organs is comparable to the embryonic state of a 

 horse, sheep, or any of the quadrupeds, why may we not then 

 have embryonic eg-g-s which correspond to the earliest stage of the 

 more highly developing eggs ? Such, probably, is the state and 

 relations of the eggs (figs. 10, 11) of Balbiani's Spirostomum 

 and Stentor, when compared with those of Laomedea (fig. 14), 

 the Rabbit (fig. 13), and Sow (fig. 12). 



From this latter point of view, then, we may look upon the 

 egg as theoretically a bipolar aggregation of albuminous and oily 

 substances, and which eventually exhibits a more or less elevated 

 degree of animality ; sometimes attaining to an eminent status, 

 as when it develops into the most highly organized animals, and 

 in other instances not rising above a very low degree, hardly 

 beyond the egg-stage, properly speaking. 



In the latter category Amoeba is found, and in fact all Rhi- 

 zopoda, as you have already been made aware of (p. 9). 

 But let us go on a little further and see how, as we ascend the 

 scale of being, the animal organization proceeds to develop be- 

 yond that degree of simplicity which obtains in the egg. 



The Sponge is a fair example of those forms which stand, in a 

 transitionary condition, between the Rhizopoda and the next 

 group of animals above them ; and we will therefore take it to il- 

 lustrate the first step in the progress toward a higher state of 

 organization. Although it is difficult to determine whether 

 Sponges are single or compound individuals, it does not affect 

 the question of their relations to the Rhizopoda. That they are 

 undoubtedly above the latter cannot be disputed when we con- 

 sider that they have a higher degree of specialization, not only 



