64 MARCUS M. HARTOG. 
Wesee that these processes fall into several distinct classes, 
which cannot be at all homologised morphologically or phy- 
siologically. (a) is a type quite apart; (6) (c) and (d) are 
modifications of one and the same process; (e) (f) and (g) 
again may be grouped together, though a nucleus is lost in 
(g), but not in the two other types; (h) (2) and (A) again form 
a similar group united by homoplasy, to use the term that 
Lankester introduced to denote the similar products of similar 
physiological conditions, irrespective of common origin ; (/) is 
distinct, showing a certain analogical relation to (a); (m) 
stands alone, and so does (n); (0) is only brought into the 
present relation because of the false homologies to which it 
has given rise. 
15. From the above it follows that excretion of protoplasm is 
no essential condition of gametogeny.! 
VII. Tue Causes or ProtopLtasMic SENESCENCE AND 
UxtimatE REPRODUCTIVE INCAPACITY. 
Maupas has established an important fact which sheds a 
flood of light into a very dark corner of biology. In the 
Ciliata, the offspring of a long-continued series of fissions 
ultimately degenerate, and lose first the power of entering 
into the conjugation that would rejuvenate them, and finally 
that of further fission. This degeneration he terms SENEs- 
CENCE. We have evidence on all sides to show that 
asexual reproduction, colonial or cellular, is rarely con- 
tinued indefinitely in those organisms which have a sexual 
process. After a certain continuance of asexual reproduction 
the strain deteriorates, as Andrew Knight showed a century 
matozoa in many mammals, where only one fertilises the single ovum. Yet 
no one has suggested that the others are excretion products. 
1 From the above summary it is obvious that Waldeyer fails utterly in his 
contention that Biitschli’s identification of the formation of polar bodies 
“must fall to the ground if it be established that sperm mother-cells also 
give rise to polar bodies,’ since the “polar bodies” or “excretions’’ in 
spermatogeny are neither universal nor fully homologous with one another, 
nor with the formation of polar bodies in the egg. 
