1895.] Dr Gaskell, The Origin of Vertebrates. 45 



region ; a proliferation which forms a solid rod of cells entirely 

 occluding the opening from the branchial chamber to the anterior 

 intestine, and then, by a hollowing out process beginning at the 

 posterior end, forms ultimately the new gut of the Petromyzon. 



Mr Bateson objected that the neural plate of the Arthropod is 

 derived from the ventral epiblast and the gut is dorsal to it ; on 

 the contrary, in the Vertebrate the central nervous system is 

 derived from the dorsal epiblast, — including in the term central 

 nervous system both the nervous layers and the layer of the central 

 canal, — and ventral to it is the notochord and vertebrate gut. This 

 difficulty seems to me more apparent than real ; the nerve layer 

 in the Vertebrate as soon as it can be distinguished is always 

 found to lie ventrally to the layer of epiblast which forms the 

 central canal. In the middle line of the body, owing to the 

 absence of the mesoblast layer, the cells which will ultimately form 

 the notochord and those which form the central nervous system 

 form a mass of cells which cannot be separated in the earlier 

 stages. The nerve layer in the Arthropod lies between the ventral 

 epiblast and the gut, the nerve layer in the Vertebrate lies 

 between the so-called hypoblast (i.e. the ventral epiblast of the 

 Arthropod) and the neural canal (i.e. the old gut of the Arthropod). 

 The new ventral surface of the Vertebrate in the head region is 

 not formed until the head fold is completed ; before this time 

 when we watch the vertebrate embryo lying on the yolk, with its 

 nervous system and central canal formed and its lateral plates of 

 mesoblast, we are watching the embryonic representation of the 

 original Limulus-like animal ; then when the lateral plates of 

 mesoblast have grown round and met in the middle line to form 

 the new ventral surface and the head fold is completed, we are 

 watching the embryonic representation of the transformation of 

 the Limulus-like animal into the Scorpion-like ancestor of the 

 Vertebrates. 



Mr Bateson and Mr Shipley object that my theory necessitates 

 enormous jumps, both in conformation and function ; I feel sure 

 upon further consideration they will see that this is not so, and 

 that in fact the main jump is not, as Mr Bateson humorously put 

 it, the sudden transformation of a Limulus into an Ammoccetes, 

 but rather the transformation of a Limulus into a Scorpion-like 

 animal by the meeting of the branchial appendages in the middle 

 ventral line. My whole argument and the whole of my work show 

 that Ammoccetes is closely allied to a Scorpion-like animal such as 

 Thelyphonus and Eurypterus, the Limulus-like animal being the 

 more remote ancestor of all three. Messrs Bateson and Shipley's 

 objections therefore are not directed against my theory of the 

 origin of Vertebrates, but against the current view of the manner 

 in which Scorpions arose from Limulus-like forms ; a view which 



