44 SCIENCE AND PHILOSOPHY OF THE ORGANISM 



order to make this point as clear as possible, I shall now 

 add to our description a few words about what may be 

 called a comparative descriptive embryology. 



COMPARATIVE EMBRYOLOGY 



Even the cleavage may present rather different aspects. 

 There may be a compact blastula, not one surrounded by 

 only one layer of cells as in Echinus ; or bilaterality may be 

 established as early as the cleavage stage as in many 

 worms and in ascidians and not so late as in Echinus. 

 The formation of the germ layers may go on in a different 

 order and under very different conditions : a rather close 

 relative of our Echinus, for instance, the starfish, forms 

 first the endoderm and afterwards the mesenchyme. In 

 many cases there is no tube of cells forming the " endoderm," 

 but a flat layer of cells is the first foundation of all the 

 intestinal organs : so it is in all birds and in the cuttlefish. 

 And, as all of you know, of course, there are very many 

 animal forms which have no proper " larval ' stage : there 

 is one in the frog, the well-known " tadpole," but the birds 

 and mammals have no larvae ; that is to say, there is no 

 special stage in the ontogeny of these forms which leads an 

 independent life for a certain time, as if it were a species 

 by itself, but all the ontogenetical stages are properly " em- 

 bryonic " the germ is always an " embryo " until it becomes 

 the perfect young organism. And you also know that not 

 all skeletons consist of carbonate of calcium, but that there 

 are skeletons of silicates, as in Eadiolaria, and of horny 

 substance, as in many sponges. And, indeed, if we were to 

 glance at the development of plants also, the differences 



