367 
+ Dr. O. Fr. von Moellendorff in Manila (Coll. Mus. Senckenberg, 
… No. 1017, 3a). 
4. Notes on the Early Stages of Segmentation in Petromyzon marinus, L. 
(americanus, Le S.). 
By Charles F. W. McClure M.A., Instructor in Biology, Princeton, College, U.S.A. 
eingeg. 13. Juli 1893. 
Although the development of Petromyzon has been carefully 
studied by the most competent investigators, there seems to be a lack 
of unity in their results; whether this is due to the histological me- 
thods employed, or to the idiosyncracies of the investigators, it is 
difficult to decide. 
In regard to the first two planes of cleavage, so far as I am aware, 
all investigators are agreed (Max Schultze!,Owsjannikow?2, Nuel?, 
Shipley4 and Kupffer‘) except Calberla®, that they are meridio- 
nal and at right angles to each other. 
Calberla stated that the first plane was equatorial and that the 
first two blastomeres were of unequal size. Scott? who studied Cal- 
berla’s material after the latters death, states that he found among 
Calberla’s material many ova in which the first plane of cleavage 
had divided the ovum into two unequal halves, but that this did not 
appear to be constant among all, so that in all probability the ova on 
which Calberla based his observations were abnormalities. 
I think it is conceded by all that the third plane of cleavage oc- 
curs normally as an Equatorial one, in every ovum where total 
cleavage is the rule, with the exception perhaps of the ova of the 
Ctenophorae®, where the third plane is represented by two Meri- 
dional furrows which form an angle of 45 degrees with each of the 
first two furrows. 
My observation on the development of Petromyzon marinus have 
led me to the conclusion that in this species at least, the third plane 
1 Die Entwicklungsgeschichte von Petromyzon Planeri. Haarlem 1856. 
2 Die Entwicklung von Petromyzon fluviatilis. Bulletin de l’Acad. d. Sc. de 
St. Petersbourg, T. 14, 1870. 
3 Developpement du Petromyzon Planeri. Archives de Biologie. Tom, II. 1881. 
4 Development of Petromyzon fluviatilis. Quart. journ. of Micr. Science. 1887. 
5 Die Entwicklung von Petromyzon Planeri. Archiv f. Mier. Anatomie. 35. Bd. 
1890. 
6 Morph. Jahrbuch. 3. Bd. p. 246. 
7 Beiträge zur Entwicklungsgeschichte der Petromyzonten. Morph. Jahrb. 
7. Bd. 1882. 
8 A. Aggasiz, Embryology of the Ctenophorae. Cambridge U.S. A. 1874. 
