GERMINAL MEMBRANES. 219 



closely compared with the mouth of the polype and 

 medusa, and the great central cavity of the sponge with 

 the stomach of the others, of the canal system with 

 the canals and cavities of the Coelenterata, — then, in 

 combination with the host of other facts, implying and 

 supporting the doctrine of Descent, the inference is 

 inevitable that in the Gastrula we have a testimony 

 of the consanguinity of the Spongiadae and Coelen- 

 terata. But this Gastrula reappears in the Holothuria ; 

 hence in the Echinoderms, in the Sagitta, in the 

 Ascidians, which will be more narrowly examined in 

 the pedigree of the Vertebrata, and finally in the 

 Lancelet; and we, therefore, hold ourselves justified 

 in regarding this coincidence of the earliest states of 

 development in difi"erent families, as the remnant of 

 the common root, which in other families, as in the 

 Articulata, for example, has been lost in the cur- 

 tailment of development. The significance of the 

 " germinal membranes " in the Vertebrata was recog- 

 nized even by Pander, and in the suggestive works of 

 V. Baer ; the extension and application of this observa- 

 tion to the whole animal kingdom, for which we are 

 especially indebted to Kowalewsky, marks one of the 

 greatest advances in the science of comparative de- 

 velopment. 



The reader unacquainted with the detailed researches 

 of our science, has already been called upon to observe 

 that there are opponents of the theory of selection, such 

 as Owen, who nevertheless accept the doctrine of Descent 

 as incontestable. Even rejecting natural selection, the 

 parallelism of Ontogenesis with Phylogenesis may also 

 be brought into the natural connection maintained by 



