164 bulletin: museum of compaeative zoology. 



is a fixed one from a stage with seven somites, until the plate no longer 

 exists as such. That it corresponds with the posterior boundary of neu- 

 romere VI of my figures, I am able to state with equal positiveness. 

 Not having found, as Locy has done, eleven segments in the cephalic 

 plate, by counting which one could determine the limits of the plate, I 

 have been obliged to resort to other means. My method of determina- 

 tion has been as follows. As a fixed point in all the. stages examined, I 

 have taken the mesodermal somite marked 7 in Plates 1 to 4. This, as 

 I determine, is the most anterior somite which becomes innervated by a 

 ventral spinal root ; it therefore corresponds, I believe, with van Wijhe's 

 7th somite.^ Anterior to this is formed a somite (van Wijhe's 6th), which 

 in early stages possesses embryonic muscle fibres, but never becomes 

 innervated by a motor root. Rabl ('92) said he could affirm with confi- 

 dence that the somite (Urwirbel) which van Wijhe holds for the 6th or 

 7th head segment in an embryo with 48 somites is identical with that 

 which he counts as the first trunk segment in an embryo of 76 

 somites. This mistake [^] of van Wijhe's, the accuracy of whose 

 work in general is so well known, has led me to take especial pains to 

 verify the identity of somite 7 in the stages most carefully examined, 

 viz. from the stage with 6 to 7 somites, until after the neural tube is closed. 

 Its identity has been determined as follows. I have carefully measured 

 the distance from the constriction between van Wijhe's 2d and 3d 

 somites — the mesodermic constriction which appears above the hyo- 

 mandibular cleft — to the partial constriction anterior to van Wijhe's 

 6th somite. This distance measured in over two hundred embryos by 

 means of camera-projection images, I have found to be practically con- 

 stant, since it increases only very slightly as the embryo increases in 

 length. Having thus determined the identity of this somite in suc- 

 cessive stages, fhave had a safe starting point for the determination of 

 the posterior limit of the cephalic plate. I have measured the distance 

 from the posterior cleft of van Wijhe's 7th somite, in the manner de- 

 scribed above, to the posterior boundary of the widely expanded cephalic 

 plate, and I have found this distance also to be constant. I chose to 

 measure from the posterior boundary of van Wijhe's 7th somite, because 

 by the measurement of this rather than a less distance the chances of 

 error were diminished. The reader can verify the constancy of this dis- 

 tance by measuring the Figures (3 to 10) on Plates 1, 2, and 3, which 

 were drawn with the aid of a camera, and are magnified forty-three 

 diameters. This distance is almost precisely the same as the distance from 

 1 Van Wijhe's 1^ occipital Somit. Rabl's 3^ distale Urwirbel. 



