MUSEUM OF COMPAKATIVE ZOOLOGY. 181 



neurotneres is not less striking than the constant relation which the 

 first, third, and fifth neuromeres bear to the fifth, seventh, and ninth 

 cranial nerves. 



The constrictions which in the chick divide the neural tract into 

 cerebral vesicles or "neuromeres," appear even before the neural walls 

 have formed a closed canal. They arise successively from before back- 

 wards, dividing the medullary tube into vesicles, which decrease in size 

 in the order of their formation. The third neuromere of the medulla 

 (fifth vesicle formed) is the only exception to the gradual diminution of 

 the successive vesicles. This neuromere (Fig. 17, Plate II.) is smaller 

 than either the second or fourth. 



Anterior to the first protovertebra the neural canal is divided by a 

 series of such constrictions into seven vesicles. With the appearance 

 of each successive protovertebra, another constriction occurs opposite 

 the protovertebra, and another neuromere (as we may also designate 

 these neural swellings) is added to the preceding. The constrictions of 

 the spinal cord often appear before the formation of the corresponding 

 protovertebrse ; consequently these ectodermic differentiations are quite 

 independent of any mechanical influences consequent upon the formation 

 of the protovertebrae. 



Beraneck has already carefully described and figured the medullary 

 folds, and in as far as the folds themselves are concerned I can but 

 confirm the results of his investigations ; yet I have ventured to repeat 

 the description in brief, because I would call attention to the fact, that 

 in the chick, at least, the order in which the various parts of the brain 

 are formed does not correspond with what is usually given as the typical 

 development of the vertebrate brain. 



Wiedersheim says : " At a very early stage three swellings may be 

 seen on the anterior enlarged part of the medullary tube, which are 

 spoken of as the primary anterior, middle, and posterior cerebral vesicles 

 (fore-, mid-, and hind-brain). . . . The primary fore-brain and hind- 

 brain each become differentiated into two parts, and thus five divisions 

 of the brain may be distinguished. Counted from before backwards, 

 these are Prosencephalon (secondary fore-brain), Thalamencephalon 

 (primary fore-brain), Mesencephalon (mid-brain), Metencephalon (sec- 

 ondary hind-brain), and Myelencephalon (primary hind-brain)." 1 I 

 quote from Wiedersheim, because he expresses so concisely the opinion 

 commonly advanced by zoologists in regard to the formation of the 

 vertebrate brain. 



1 Comparative Anatomy of Vertebrates, pp. 131, 132. 



