60 . LOWER INVERTEBRATES. 



condition, and this would give the vase-shaped Ascoiies. The inheritance of the in- 

 vaginated stage or of the primitive differentiation of the colony into protective and 

 feeding zoons in any encysted egg form would be necessarily attended by the forma- 

 tion of a globular shape in which one end would have cells of a different kind from the 

 other, one bemg composed of endodermal cells inheriting the digestive functions of the 

 original colony, while the other would be formed of ectodermal cells arising from the 

 protective zoons. This encysted form would be composed of but one layer of cells, and 

 therefore have a hollow interior, and the supporting zoons or meso- 

 derm would be formed between the other two membranes when it 

 became necessary by the protozoon method of reproduction by 

 fission. 



AVe can also reverse this explanation and imagine a sponge, 

 one of the Ascones, being reduced to a protozoon ; losing first the 

 form, then the supporting layer, then the protecti^■e cells, and finally 

 becoming converted into a layer of zoons, each of which would 

 closely resemble those to be seen in Fig. 29. The validity of this 

 comparison may be seen by comparing this figure of Codosiga with 

 Fig. 53. — Flagellated the flagellated ampuUaceous cclls of a true sponge shown in Fig. 

 of Sycaiirira."''" * 52 ; and the comparison will also gain when we recollect that in 



the young of the flagellated protozoon the stalk is absent. 

 The normal action of the law of concentration and acceleration of development 

 would alone ha\e caused such changes in the modes of growth of the Metazoa if the 

 latter were really the descendants of the Protozoa, and this series of transformations 

 is included when we say that the 3Ietazoa, in accordance with this law, have inherited 

 the tendency to form colonies or tissues by fissiparity, at an early stage in the exis- 

 tence of the cell or zoon. Thus, the individualized protozoanal stage has become con- 

 fined to the earliest periods of existence instead of being more or less permanent and 

 characteristic of the later stages of growth as in the Protozoa. When the colony be- 

 comes embryonic the process of multiplying by division is, as a necessary consequence, 

 also accelerated and concentrated, and tissues are rapidly formed for different pur- 

 poses. We can therefore, without calling to our aid any but the well-known effects of 

 habit and the law of concentration of development, account for the segmentation of 

 the egg, and the subsequent tendency of the j^rimitive tissue to give rise to the three 

 layers of the Metazoa. The fact that certain cells become differentiated into eggs, and 

 others from the same or other layers, into spermatozoa, is not more remarkable than 

 that certain zoons of many colonial Metazoa, like the hydroids, are exclusively egg- 

 bearers, while others are solely sperm-bearers. The fact that the male element seeks 

 out the egg and becomes merged in it, is paralleled by the jirocess of conjugation among 

 the Protozoa. The sperm cells of the IMetazoa, like the germs of the Protozoa, arise by 

 division of a single cell, and, although frequently of similar shape to and swimmino- 

 freely like many protozoon germs, they do not wait until maturity before conjugating 

 with the females. This is plainly only the inheritance of the tendency to conjugate 

 at an earlier stage, and is a natural result of the law already laid down. 



It is well known that there is a tendency to reproduce after conjugation, and that 

 conjugation is performed by Protozoa of different sexes, and also that there are 

 sexual colonies among the higher Protozoa. The result of differentiation or progress 

 is evidently towards the formation of sexual differences in the Protozoa as in other 

 branches of the animal kingdom, and if our view is correct, we ought to expect that 



