366 WILLIAM PATTEN. 



that regards the " primitive streak " and " blastopore " as 

 remnants of a gastrula — unless, indeed, we expect to prove that 

 the Vertebrates out-Coelenterate the Coelenterates. The ab- 

 sence of an Annelid pre-oral lobe, and the formation of the 

 head by the pushing forwards of three post-oral body segments, 

 show that the Arthropod head and body are comparable only 

 with the post-oral portion of the Annelid. The Arthropod 

 body represents an outgrowth from the trochosphere, but the 

 trochosphere itself, the Coelenterate stage, has disappeared. 

 Hence there is no such thing as a gastrula in Arthropods, and, 

 strictly speaking, no germ layers. The germ-layer theory 

 requires, as one of its ablest expounders, Balfour, explicitly 

 states, that the entire ectoderm, mesoderm, and endoderm be 

 derived directly from the primitive layers. Now in Lopado- 

 rhynchus it is certain that the greater part of the mesoderm 

 arises from the ectoderm at the growing tip of the tail, and has 

 nothing to do with primitive mesoderm. In Arthropods the 

 mesoderm and also part of the endoderm may arise in an 

 exactly similar manner. Besides, mesoderm may arise (Aci- 

 lius) at a late embryonic period from a great variety of places, 

 just as ganglionic cells are formed from the general surface of 

 the body wherever a new sense-organ is formed. Hence it is 

 highly probable that all the endoderm, except possibly a small 

 portion at the inner end of the oesophagus, and the mesoderm 

 have arisen independently of, and have finally supplanted, the 

 primitive layers ; just as in Arthropods the pre-oral lobe and 

 brain of Annelids have been replaced by other organs. 



If we suppose that the body of segmented animals repre- 

 sents, not the elongated body of a Coelenterate, but only one 

 of its tentacles, we can explain why a segmented animal 

 always grows at one end only — something the concrescence and 

 other theories cannot do ; we can also explain why the Annelid 

 and molluscous body is a lateral outgrowth from the subum- 

 brella ; and this supposition would be in perfect harmony with 

 the position and history of the '' Prototroch " nerve, as de- 

 scribed by Kleiuenberg. Moreover, the imperfect division of 

 Coelenterate tentacles into joints provided with lateral pro- 



