214 
BULLETIN OF THE UNITED S'l'ATES FISH COMMISSION. 
Such a one is given in Fig. 7. Here both cells of one lateral pair {I and r) have 
deviated from the type and are undergoing meridional division, and both cells of the 
opposite pair are doing likewise. This state of alfairs is, however, not the rule, and 
serves rather as a suggestion than an argument. But what is the rule, is the exact 
agreement between the corresponding cells of opposite halves as to the time when 
cellular fission begins. The furrows appear at jirecisely the same instant in r and r' 
and I and //, anti the same vtas true of the terminal cells m, m' and «, n'. In the ordi- 
nary blastoderm, as shown in Fig. G, all the furrows start at exactly the same time. 
Such correspondence between the opposite halves plainly suggests that the bilaterality j, 
is not simply a morphological one, but that the symmetrical arrangement of cells is 
the mere outward expression of a physiological bilaterality which already exerts a 
control over the life of the organism. 
Watase has shown in his careful study of the Squid segmentation (41 and 42) that 
the bilaterality is there even more prominent than it is in the Teleost. If, for instance, 
a cell on one side exhibits such a variation as multinuclear fission, the corresponding' 
cell on the opi)osite side will do the same. The relation between the right and left 
halves of the blastoderm in regard to the time when activity begins is, however, of a 
very different character in the two animals. In the Squid there is an alternation in i 
the activity of the two sides ; one side undergoes nuclear division while in the cells of 
the opposite side the nuclei remain in the resting stage. In the Teleost, on the con- j 
trary, there is coincidence between the two halves of the blastoderm as regards activity '! 
and rest. This is certainly true of the cellular division, and without having studied I' 
the nuclear fission in anything like the detail which characterizes Watase’s work on 
the Squid, I feel sure that the same coincidence is present in this form of activity as 
well. I never found resting nuclei on one side and amphiasters on the otlier, but the 
resting nuclei, sharply outlined and conspicuous, always made their appearance on i 
the two sides at the same time, and as soon as amphiasters could be recognized on one 
side they could be found on the other. 
Ami)Mhian and Teleostean segmentation . — The teleostean segmentation has un- 
doubtedly been derived from a total segmentation essentially like that of Amphibia, i 
and, convinced of this, Eauber (36), Agassiz and Whitman (1), and Ziegler (47) have 
endeavored to homologize the early furrows in the two groups. In regard to the first 
two furrows there can be no difference of opinion. The homology of the third teleos- 
tean furrow is, however, by no means so clear. Ziegler, without entering into a 
detailed discussion of the matter, regards the first three furrows in the two groups as 
homologous. Agassiz and Whitman, after a critical examination of Eauber’s views, 
also pronounce in favor of this homology, deciding that the third teleostean furrow i 
represents the equatorial furrow of Amphibia. I do not find, however, their reasons 
sufficient for discarding the homology olfered by Eauber, supiiorted as it is by varia-| 
tious (atavistic) in the teleostean germ towards the Amphibian type, and by variations 
in the Amphibian segmentation which so exactly imitate the teleostean type. 
In his extremely suggestive paper Eauber (3G) shows that the generally accepted 
account of the frog’s segmentation is erroneous. The furrows have not the ideally 
symmetrical arrangement given in the common figures. It is very rare even for the 
second meridional furrow to cross the first at the poles of the egg, and Eauber never 
observed an egg in which the first three meridional furrows so cross. Commonly the 
two furrows, which together compose the so-called second meridional furrow, meet the 
