Lloyd-Jones: Mules That Breed 
vations are in line with those of Guyer 
(00) on sterile hybrid pigeons. Iwanoff 
further reports abundant parenchyma 
and interstitial cells, and emphasizes 
the secretory nature of these tissues, 
holding them accountable for the second- 
ary sex characters of the mules. 
Whitehead (708) also reports work on 
testis structure inmules. He found no 
secondary spermatocytes and no sperma- 
tozoa of any kind; interstitial cells were 
abundant and granular. Whitehead 
had examined the testes of cryptorchid 
horses and found them to’be similar in 
structure to those of the mule. He 
puts the scrotal testis of the mule in 
the same category with the abdominal 
testis in cryptorchids of pure species. 
Sexual passion is quite apart from sexual 
fruitfulness, mules and cryptorchids 
exhibiting the former in marked degree. 
Like Iwanoff, Whitehead says that 
sexual passion is due to specific internal 
secretions of the interstitial cells. ‘‘ They 
are the only cells which can elaborate it 
for the only other secretory cells are 
degenerate, while the interstitial cells 
are hypertrophied.’’? 
The most recent, and detailed work 
on the structure of the mule testis is by 
Wodsedalek (716). He has carefully 
followed the cell cycle in the mule testis, 
and, carrying out the suggestions of 
Guyer made for pigeons in 1900, has 
offered a specific explanation for the 
abnormalities seen in the testis, which 
result in sterility. Wodsedalek had 
previously reported (’14) thirty-eight as 
the chromosome number in the mare— 
each ovum carrying the haploid number 
nineteen. From his study the above 
author concludes that a_ plausible 
chromosome number for the jack (male 
ass) is much greater than the horse, 
namely sixty-five, each sperm carrying 
thirty-two or thirty-three. As a con- 
sequence the fertilized egg destined to 
produce a male mule shows fifty-one 
chromosomes, nineteen from the mare 
and thirty-two from the jack. (Itisas- 
sumed that the female mule would show 
fifty-two.) Despite the great diversity 
in the nature and number of the chromo- 
somes contributed by the ovum of the 
501 
mare on the one hand, and sperm of 
the jack on the other, mitosis and 
cleavage processes are apparently undis- 
turbed, for growth and development of 
the fetus and of the foal proceed nor- 
mally. 
CAUSE OF STERILITY 
As fat as can be seen there is nothing 
irregular in the growth or cell divisions 
of the mule till he reaches sexual ma- 
turity. Up to this time maternal and 
paternal chromosomes have lain side by 
side and carried on their customary 
functions. There has been no necessity 
for them to cooperate to any noticeable 
extent, each chromosome has divided 
at mitosis independent of the others, 
one-half going to each daughter cell. 
The real conflict ensues during the 
various stages of the primary sperma- 
tocyte. Normally at this stage there 
is a pairing and subsequent separation 
of homologous chromosomes from father 
and mother. In case of the mule, how- 
ever, because the ovum and sperm con- 
tributed such unequal numbers of 
chromosomes, there are many chromo- 
somes without a homologue with which 
to mate, and even in case of homologues 
the physiological incompatibility of the 
two plasms render the pairing difficult 
and incomplete, or prevents it entirely. 
“So great is this disturbance that the 
destruction of each cell is inevitable 
and no spermatozoa are produced, 
causing the hybrid to be sterile.”” It is 
plain that the evidence drawn from such 
studies makes the likelihood of these 
animals begetting offspring extremely 
small if not altogether negligible. 
However, it would seem wise to be 
conservative and tolerant. Several cases 
have been reported where there is a 
regular and orderly disappearance of 
sperm-forming cells. For instance, 
Morgan has shown that in certain gen- 
erations of Phylloxerans, in spermato- 
genesis, half the spermatids (those lack- 
ing the accessory chromosome) regularly 
degenerate. The disappearance of these 
cells is precise in nature, and occurs in an 
orderly fashion at a fixed stage in the 
cycle of divisions. Apparently, how- 
2 Miss Boring (Biol. Bull., XXIII, pp. 141-153), working on the testis of the fowl, failed to find 
evidence in support of the view that interstitial tissue is responsible for secondary sex characters. 
