OUR EMBRYONIC DEVELOPMENT. 59 



of the seed-filaments of the sponge in 1866. Thus it was 

 shown that both the materials of generation, the male 

 sperm and the female ova, fell in with the cellular 

 theory. That was a discovery of which the great 

 philosophic significance was not appreciated until a 

 much later date, on a close study of the phenomena 

 of conception in 1875. 



All the older studies in embryonic development 

 concern man and the higher vertebrates, especially 

 the embryonic bird, since hens' eggs are the largest 

 and most convenient objects for investigation, and are 

 plentiful enough to facilitate experiment ; we can hatch 

 them in the incubator, as well as by the natural 

 function of the hen, and so observe from hour to 

 hour, during the space of three weeks, the whole 

 series of formations, from the simple germ -cell to the 

 complete organism. Even Baer had only been able 

 to gather from such observations the fact that the 

 different classes of vertebrates agreed in the charac- 

 teristic form of the gerra-layers and the growth of 

 particular organs. In the innumerable classes of 

 invertebrates, on the other hand — that is, in the great 

 majority of animals — the embryonic development 

 seemed to run quite a different course, and most of 

 them seemed to be altogether without true germinal 

 layers. It was not until about the middle of the 

 century that such layers were found in some of the 

 invertebrates. Huxley, for instance, found them in 

 the ruedusse in 1849, and Kolliker in the cephalopods 

 in 1844. Particularly important was the discovery by 

 Kowalewsky (1886) that the lowest vertebrate — the 

 lancelot, or amphioxus — is developed in just the same 

 manner (and a very original fashion it is) as an 

 invertebrate, apparently quite remote, tunicate, the 



