14 GENERAL EMBRYOLOGY. 
It is both interesting and significant that in some cases the first polar body divides simul- 
taneously with the separation of the second polar body, each of its segments containing the same 
number of chromosomes as the female pronucleus. This condition, indeed, is by no means 
uncommon amongst invertebrates. It occurs in Ascaris, in certain crustaceans, and in some 
molluscs. Amongst vertebrates, however, the division of the first polar body, so far at least as 
present observations go, must be regarded as exceptional. It has been seen to occur in bats by 
Van Beneden and Julin, and in mice by Tafani and Sobatta, but apparently even in these it 1s 
not a constant phenomenon. As regards the spermatozoa also, though it is known that they are 
formed by the division of sperm-mother cells, the exact details of their development in mammals 
have not yet been satisfactorily elucidated. It seems, however, very probable that in the higher 
forms of animal life, as in the lower, the processes of maturation of the ovum and the formation 
of the spermatozoa are simply those of cell-division associated with a reduction of the number of 
chromosomes in the resulting grand-daughter cells ; these latter being represented in the higher 
forms, including mamials, on the male side by the spermatozoa, and on the female side by the 
second polar body and the mature ovum, except in the apparently occasional instances in which 
the first polar body divides, and in this case there are three polar bodies and the mature ovum. 
A spermatozoon, like the ovum, is a nucleated mass of cytoplasm, but it 
presents striking modifications in structure. It is very small, and possesses a 
head, a neck, a body, a tail, an 
end-piece, a spear, an axial fila- 
Head ment, a spiral filament, a spiral 
membrane, a head-cap,and acer- 
Body tain amount of protoplasmic sub- “” 
stance called the protoplasmic cia 
remnant. The more obvious of Head cap 
a these, namely the head, body, Heaa Contrahiaay 
al 
long filamentous tail, and thin 
end-piece, have long been re- 
cognised as essential elements of 
a spermatozoon (Fig. 6), and it Boay 
is only comparatively recently 
that the remaining parts have 
been specially described. The 
head is ovoid, and laterally com- 
pressed, so that when viewed 
from the side it appears pointed; 
Protoplasmie 
Neck remnant 
Axial filament 
Spiral filament 
End piece— 
B it is about 4°5 » long, 2°5 p broad, Spiral mem- 
5] alas ee 4 ‘a Tail ———-# brane with 
PEE ie Guta endean | Gand Gi5h px thick. It consists of h Taare 
(after Retzius). two parts—an anterior clear,and filament 
A, Side view ; B, Front view. @ posterior more stainable and 
transversely striated. Within 
the head is a central body which is clear, refractile, and 
not easily stainable ; in stained specimens it is sometimes 
marked by coloured lines, and occasionally it is divided 
into several pieces. The head is traversed by the axial 
filament, and it is surrounded by the clear head-cap, 
within which is a thin protoplasmic layer; this is con- 
tinuous posteriorly with the protoplasmic remnant which 4,4. _ 
surrounds the neck. piece 
The neck is a very short constricted portion uniting 4, + _gepyorurE oF A 
the head and body. It is clear, it is traversed by the gprrmarozoon (Diagrammatic). 
axial filament, it sometimes contains a rounded body, . i 
the globuloid body, and it is surrounded by the protoplasmic remnant, which 
latter varies considerably in amount. 
The body is somewhat longer than the head, it also is traversed by the axial 
filament, it is encircled by the spiral filament, and it gives attachment to the spiral 
membrane. 
The tail or flagellum is about six times the length of the body, it may terminate 
in an enlargement or it may taper towards its extremity, the axial filament lies in 
its centre, and the spiral filament and spiral membrane are coiled around it. 
Projecting from its posterior end is a fine filamentous process, the end-piece, which 
