THE HISTOLOGY OF THE BLOOD OF LARVA OF ZEPIDOSIREN PARADOXA. 303 
It would be beyond the scope of this paper to enter on the possible theories as to 
the changes underlying the retractive phenomena in the protoplasm, but it may be noted 
_ that as the centrosome does not enlarge during mitosis, there can be no actual centri- 
petal movement of the protoplasm on to that body. The same difficulty presents itself 
if we supposed, with Ruumsuer, that the centrosome acts by the abstraction of water 
from the hyaloplasmic framework, causing it to thicken and condense, unless it were 
further supposed that the water entered into new combinations in the centrosome, which 
it is not very easy to accept. 
The account I have given above is an attempt to explain merely the phenomena as 
they are presented in the individual case, and does not involve a general theory of the 
mechanism of mitosis. It seems at first sight radically different from that given by 
various observers (Wixson,* TrIcHMANN,t and others) of the appearances in dividing 
ova, in which the radiations are conceived as manifestations of an actual centripetal move- 
ment of the hyaloplasm. It may, however, be that the contradiction is one of appear- 
ance only. ‘The essential factor is the same in both cases—a centripetal condensation of 
the hyaloplasm. In very fluid protoplasm like that of the ovum, there may well be an 
actual centripetal movement; but in very viscous protoplasm like that of the red 
corpuscles, which are undoubtedly firm and elastic bodies, the condensation may 
involve only retraction without a flowing movement. If the framework is fixed peri- 
pherally the retraction would involve increased tension and the rays would become 
contractile fibrils. Thus no one explanation will apply to all cases; for if the centrosome 
and its radiations are the expression of a condensation of the active protoplasm, due to 
chemical or physical causes, the mechanical results will vary with the consistency of 
the medium in which such condensation occurs. 
Il. SrructurRE OF THE LEUCOCYTES. 
Though it is now well known that in all classes of vertebrates the blood of the adult 
contains leucocytes of several different varieties, showing very different reactions to 
various dyes, little is known about the first appearance of the white elements in the 
blood of the embryo. ‘The stage of embryonic life at which they appear seems to vary. 
In Lepidosiren the blood is already at a very early stage provided with several different 
kinds of leucocytes, but in the present writing I shall describe merely the morphology 
of the different kinds of free cells I have found in the blood and tissues of the embryo, 
reserving for a future communication the questions regarding the origin of the different 
varieties, and the interrelation between them. 
(1) Small Mononuclear Hyaline Corpuscles. 
This form occurs sparsely in the blood, but more abundantly in the spaces adjoining 
the posterior cardinal sinus. 
* Arch. f. Entwickelungsmek., Bd. xiii. + Ibid., Bd. xvi. 
