184 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 
cephalomeres, since they later subdivide into segments which dorsally 
are equally long and broad. The Vorderhirn divides into two encepha- 
lomeges, the Mittelhirn into three, and the Hinterhirn into three. Thus, 
since the posterior five “primáre Abschnitte” 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 8. 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 Hinterhirn ; the last three are hindbrain neuromeres IV, V, 
and VI. Since Zimmermann’s 7th and 8th “primäre Abschnitte ” are 
not differentiated at this stage, Iam unable to accept his conclusion that 
there are at first eight primary “ encephalomeres ” or “Abschnitte.” The 
primary forebrain subdivides into the two dorsal expansions which Zim- 
mermann calls “ Secundäre Vorderhirn ” and “ Zwischenhirn.” But, if 
these are “encephalomeres,” I am unable to see how later differentia- 
tions, such as the prosencephalon (epencephalon), paraphysis (paren- 
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 “ Secundäre Vorderhirn ” and “ Zwischen- 
hirn ” 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 affecting its ventral and 
lateral walls only, 
The third vesicle, Zimmermann’s Hinterhirn, which he says subdivides 
into three “ Encephalomeren,” I find to become differentiated into the 
cerebellum Anlage and a posterior enlargement or thickening, but nothing 
