LINNEAN SOCIETY OF LONDON. 1 7 



Dr. Gaskell indulges in a polemic against the germ-layer theory, 

 whilst maintaining strongly the theory that tlie development of 

 the embryo recapitulates the history of the race. He seems to be 

 unaware that the germ-layer theory is only a special instance of the 

 recapitulation theory. It asserts that the egg in its progress to a 

 hollow blastula recapitulates the change from a unicellular to a 

 multicellular Protozoon. This part of it Dr. Gaskell accepts, and 

 with justice, for in the development of simple and primitive types 

 the blastula crops up throughout the entire Animal Kingdom. But 

 in every case which is free from the complication of yolk, the blastula 

 is transformed into a hollow gastrula by a process of invngination, 

 so that we find that in the Arthropod Lucifer and the Vertebrate 

 AmpJdoxus the process is very similar. And yet Dr. Gaskell asks 

 us to believe that in the one case the cavity is homologous with 

 the neural canal of Vertebrata and in the other with the gut ! 

 Such reasoning seems to me to be very difficult to accept. 

 Dr. Gaskell assumes that Lucifer developed the hollow gastrula 

 stage because its egg is a small one floating in water and has to 

 absorb nourishment early through the blastopore, but the fact 

 is that in this stage of its development the egg of Lucifer is in a 

 tough shell and that before it begins to absorb nourishment the 

 blastopore closes, and this is the case with AmpJiio.vus also. The 

 doubts as to the validity of the germ-layer theory have concerned 

 themselves chiefly with the nature of the third layer, the mesoderm. 

 If everything which is found between ectoderm and endoderm be 

 called mesoderm no doubt confusion will arise, for heterogeneous 

 structures are confounded under this name. But the more careful 

 investigation of doubtful cases in recent years, for which we are 

 specially indebted to the Americans, have shown that if by meso- 

 derm we mean the wall of the coelom, theu this is homologous in 

 all cases and always arises from the gut-wall. 



I pass over minor difficulties of Dr. GaskelTs theory, such as 

 the degeneration of the ancient genital gland into packing tissue 

 surrounding the brain, and the transformation of the womb into 

 a gland which in Ammocoetes, as in Amphioxus, produces a string 

 of mucus to entangle food ; for the mind which accepts the main 

 ideas of the theory will be capable of digesting such trifles also. 

 We come then to the only points in Dr. Gaskell's theory which 

 in the mind of any zoologist would constitute even prima facie 

 evidence in its favour, viz. the external resemblance between some 

 of the armoured flsh of the Devonian and the contemporary Eury- 

 pterids, coupled with the assertion that when Vertebrates appeared 

 Arthropods were dominant in the water, and that only creatures 

 with strong armour and well-developed nervous systems could have 

 overcome them. Dr. Gaskell infers that if the primitive Verte- 

 brates had been \\ke Amphioxus they never could have won the day. 

 Now to this, two answers may be made, first, that the resem- 

 blance is purely superficial, in fact far less than exists between a 

 Whale and a Fish : we should in fact have far more reason for 

 classing Whales as Fish than for regarding Ceplialaspis as allied 



LINN. SOC. PROCEEDINGS. — SESSION 1909-1910. C 



