596 ARTHUR THOMSON. 
that Strasburger insists on their arbitrary occurrence, dis- 
tinctly present in one form, but apparently altogether absent 
in others nearly allied; yet it is not necessary to go so far as 
he seems to do in doubting whether such elements as Bauch- 
kanalzelle and polar cells have really any definite morphological 
import, since it is easy to understand how a process of slight 
morphological import, whose persistence is of course con- 
ditioned by an immediate physiological necessity, might dis- 
appear in any case where the Bak ca necessity was other- 
wise satisfied. 
Comparison with similar Processes in Spermato- 
zenesis.—The prevalence of such physiological theories as 
those of Balfour and Minot, according to which the formation 
of polar cells is an extrusion of male elements from the ovum, 
has led to a frequent comparison between the latter process 
and the separation of elements during spermatogenesis. That 
such comparisons are not necessarily merely physiological, is 
evident from the homology between ovum and mother-sperm 
cells! (S'), first emphasised by Reichert in 1847, and since 
then more or less consistently recognised by various investiga- 
tors. The frequent close resemblance between the structure 
and origin of the glands and between the early stages of the 
sex-cells has been often noted; and, further, Fol, for instance, 
has maintained that the follicular cells are genetically the 
strict homologues of spermatoblasts; while Nussbaum 
(‘Archiv f. mikr. Anat., xviii), following von la Vallette 
St. George, collates spermatogonium with ovum, and the fol- 
licular envelope (Follikelhaut) of the spermatogonium with 
the follicular epithelium of the ovum, maintaining that in 
Amphibia and Teleostei spermatogonium and follicular cells 
arise from the morula-like division of the nucleus of a primitive 
1 In the confused state of the nomenclature of spermatogenesis, it is con- 
venient to denote the undifferentiated sperms, spermatocytes, &c., as (8°), 
the cells from the division of which these result, spermatogonia, spermato- 
blasts, &c., as (S!) the mother-cells of these (S?). See Geddes and Arthur 
Thomson on ‘ History and Theory of Spermatogenesis,’ Proc. Roy. Soe. 
Kdin., 1886. 
