184 BULLETIN: MUSEUM OF COMPAKATIVE ZOOLOGY. 



ceplialomeres, since they later subdivide into segments which dorsally 

 are equally long and broad. The Vorderhirn divides into two encepha- 

 lomeres, the Mittelhim into three, and the Hinterhirn into three. Thus, 

 since the posterior five " primare Abschuitte " do not further subdivide 

 there are in all thirteen " encephalomeres." As a result of cephalic 

 flexure some of the encephalomeres become wedge-shaped, but all are 

 clearly separated from one another by constrictions. Zimmermann's 

 paper was a preliminary one without figures, and it has not as yet been 

 followed by a final paper. 



It is seen that Zimmermann's account, based on the study of S. acan- 

 thias embryos, differs somewhat from my own. At the closure of the 

 neural tube I find six vesicles or expansions of the encephalon. The first 

 three correspond with those called by Zimmermann Vorderhirn, Mittel- 

 hirn, and Hinterhii-n ; the last three are hindbrain neuromeres IV, V, 

 and VI. Since Zimmermann's 7th and 8th " primare Abschnitte " are 

 not differentiated at this stage, I am unable to accept his conclusion that 

 there are at first eight primary " encephalomeres " or "Abschnitte." The 

 primary forebraiu subdivides into the two dorsal expansions which Zim- 

 mermann calls " Secundare Vorderhirn " and " Zwischenhim." But, if 

 these are " encephalomeres," I am unable to see how later differentia- 

 tions, such as the prosencephalon (epencephalon), paraphysis (pareu- 

 cephalon), and epiphysis can be excluded from the same category. 

 May we not have tertiary as well secondary "encephalomeres"? I am 

 unable to accept Zimmermann's single criterion of size as sufficient to 

 enable us to make a distinction between those segments which are prim- 

 itive, i. e. remnants of ancestral structures, and those which are the early 

 beginning of adult organs. A most serious objection to regarding such 

 structures as Zimmermann's " Secundare Vorderhirn" and "Zwischen- 

 him " morphologically comparable with neuromeres or myelomeres has 

 been stated by Herrick ('92), and consists in the difficulty of homol- 

 ogizing dorsal expansions with ventral ones. 



The primary midbrain, as stated by Zimmermann, subdivides into 

 three segments, the most anterior of which lies in front of the posterior 

 commissure and in front of the place of origin of the oculomotor nerve. 

 In all stages the midbrain is seen in median sagittal sections to present 

 a simple dorsal expansion, its constrictions aflfecting its ventral and 

 lateral walls only. 



The third vesicle, Zimmermann's Hinterhirn, which he says subdivides 

 into three " Encephalomereu," I find to become differentiated into the 

 cerebellum Anlage and a posterior enlargement or thickening, but nothing 



