1880. | A Sketch of Comparative Embryology. 243 
At some time during these changes, whether sooner or later 
perhaps does not matter, a single spermatozoon enters the egg. 
As the egg or yolk is surrounded by envelops, it is evident that 
either the spermatozoa must enter before the coverings are 
formed, or that it must have some way of passing them. For 
instance, in the hen the spermatozoa attain the yolk before the 
hard shell is formed in the oviduct. In other instances there is a 
special opening, often having a peculiar structure, which admits 
the passage of the spermatozoa, and is 
called the micropyle, Fig. 10. This 
opening is of course not an essential 
part of an egg, and merely permits the 
egg to be protected by an impermeable 
shell without excluding the spermato- 
zoon. It is asserted that in some cases z 
the micropyle is not a real opening, but vend Sapari EE 
only a permeable spot through which 
the spermotozoön can work its way (Kupffer). 
It was stated above that only a single spermatozoon enters the 
yolk. The way in which the entrance of a second one is pre- 
vented is not definitely determined yet. It has, however, been 
stated by Fol and by Kupffer and Benecke that in the eggs 
observed by them (star-fish and lamprey) there is no vitelline 
membrane around the egg until after impregnation, when a com- 
plete envelop is rapidly formed by the yolk, effectually excluding 
all other spermatozoa. If this view is correct, then the egg has 
no proper cell membrane until after its fertilization; and all the 
coverings it has before that event, are only secreted around it by 
other cells, and not by itself. 
When a spermatozoon penetrates into an egg, the head goes in 
first ; after which the nucleus it contains loses its peculiar shape, 
transforming itself within the yolk into a small spherical or irreg- 
ularly-shaped male pronucleus. The tail disappears—how is not 
known, This pronucleus, like the female, is surrounded by 
radiating lines, so that there is also a male aster. At this time 
the egg, as shown in Fig. 11, contains two pronuclei, and is still 
Connected with the polar globules. The second pronucleus also 
travels towards the center of the egg, where the two pronuclei — 
Meet, both having meanwhile enlarged considerably. After 
coming in contact the two pronuclei fuse completely, making a — ae 
