82 
SUMMARY OF CURRENT RESEARCHES RELATING TO 
ccelomic epithelium or peritoneum. In several respects the author’s 
results differ radically from Caldwell’s. 
Plankton Brachiopods.* — Prof. H. Simroth reports on the sparse 
collection of Brachiopod larvae made by the Plankton Expedition. There 
were only four — all Ecardinate — possibly belonging to Discina , Lingula , 
and Crania. 
Development of Myzostoma glabrum.f — Dr. W. M. Wheeler has 
studied in detail the maturation, fecundation, and early cleavage of the 
eggs of this form in their bearing upon the problems of modern cytology. 
He confirms and amplifies the descriptions of Loven and others with 
regard to the structure of the spermatozoon. This has an elongated 
contractile head, containing a series of chromatin beads, and a slender, 
motionless, and apparently inconstant tail, now described for the first 
time. The middle piece with its centrosome is apparently absent, and 
the movements are produced by the striated head. Associated with 
these peculiarities of the sperm, the author finds that the egg, at the 
time of fertilisation, is “ characterised by a certain immaturity and con- 
servatism,” an apparent lack of well-marked sexual differentiation. This 
is shown, inter alia, by the delay which occurs between the entrance of 
the spermatozoon and the response of the egg, by the long persistence of 
the nucleolus, and especially by the method of formation of the polar 
bodies. With regard to the last, the points of importance are the large 
size of the asters and centrosomes of the polar body spindles, the fact 
that the first spindle appears near the middle of the egg and not at its 
periphery, and that, in the case of the second polar body, the centro- 
somes divide early, so that two are found at each pole of the spindle. 
Further, the centrosomes at the inner pole seem to become the centro- 
somes of the first cleavage spindle. 
The general part of the paper is concerned with the following 
points : — (1) the polarity of the ovum ; (2) the function of the nucleolus ; 
(3) the significance of the polar bodies ; (4) the reduction of the chro- 
matin ; (5) the origin of the centrosomes of the first cleavage spindle. 
(1) In Myzostoma the polarity of the ovum is established at a very 
early stage, and, in spite of the pressure to which the ripening eggs are 
subjected, is never lost ; the author is of opinion that this is also true of 
the eggs of the majority of animals. (2) The nucleolus of Myzostoma , 
although unusually persistent, takes no part in the formation of the 
polar body spindles ; and, similarly, the numerous nucleoli of the pro- 
nuclei take no part in the formation of the first cleavage spindle. These 
facts, therefore, support Hacker’s views as to the secretory nature of the 
nucleolus and its unimportance in cell-division. (3) The size and de- 
velopment of the polar bodies seem to support the view that these are 
rudimentary or abortive ova. (4) There is no evidence of a reducing 
division in the formation of the polar bodies, but the author is inclined 
to believe that chromatin reduction occurs during the prophases of the 
first cleavage spindle. (5) The author argues strongly against the ex- 
istence of Fol’s “ quadrille,” but believes that Myzostoma] shows that 
the centrosomes of the first cleavage spindle may originate Neither from 
the sperm or from the egg. 
* Ergebn. Plankton Exp., ii. (1897) 17 pp. and 1 pi. Zool. Centralbl., iv. (1897) 
pp. 784-5. t Arch. Biol., xv. (1897) pp. 1-70 (3 pis.). 
