ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 565 



longitudinal coupling of chromosomes, and the separation of the couple 

 at the first mitosis. 



The maturation-divisions may be accomplished before the liberation 

 of the ova (as in the sea-urchin), or immediately after (as in Asterias) -. 

 commonest perhaps is that the spermatozoon enters after the ovum has 

 expelled the first polar body and is preparing to form the second. When 

 the spermatozoon enters, a wave of contraction, starting from the upper 

 pole, passes through the ovum ; a small quantity of fluid (perivitelline) 

 is expelled which insinuates itself between the cortex of the cytoplasm 

 and the innermost layer of the egg-membranes. The ovum swims in 

 this liquid, to which water is often added. The second maturation- 

 division then occurs. An active ovum-centrosome is in some cases — e.g. 

 J/ac^m— undoubtedly present ; in other cases — e.g. frog — it is un- 

 doubtedly absent, and there maybe intermediate conditions. 



In connexion with fertilization, Brachet discusses at length the 

 problem of monospermy. Why is it that in most cases only one sperma- 

 tozoon enters the ovum ? If several enter, as in some Insects, Rryozoa, 

 Selachians, Urodela, Reptiles, Birds, why is it that only one accomplishes 

 fertilization ? The entrance of the first spermatozoon may bring about 

 blocking of the micropyle, but many ova have no micropyle. The 

 contraction of the cytoplasm and the formation of the perivitelline 

 liquid may also prevent polyspermy. Brachet's observations on frog's 

 ova artificially rendered polyspermic (e.g. by using very concentrated 

 seminal material) show the formation of five or so spermatic energids 

 (with nucleus and centrosome and radiations) which are autonomous, 

 repel one another, and rapidly reach a condition of equilibrium. The 

 establishment of the energid renders the cytoplasm refractory to the 

 entrance of another spermatozoon. Very interesting is Riickert's 

 observation that some of the merocytic nuclei which have a transforming 

 action on the yolk of Selachian ova (the others being blastomeric) are 

 heads of accessory spermatozoa. The first spermatozoon to enter is the 

 fertilizing agent, and forms a central energid which repels the others to 

 the periphery. What happens to accessory sperm-nuclei in birds and 

 reptiles is unknown. While energids repel one another the pronuclei 

 attract one another, unless the associated centrosomes counteract this. 

 Three sperm pronuclei may attract one another in a polyspermic ovum. 



A ripe ovum passes into a state of " maturation-equilibrium " or 

 inertia, but for a time a slight metabolism — a sort of "agony"— persists. 

 Thus Loeb has proved- the occurrence of oxidations. Is the depression 

 pf the ovum comparable to that of Infusorians after many divisions ? 

 Is it due to auto-intoxication ? Is it due to " senescence " in Child's 

 sense ? Is there progressive impermeability (Lillie) or osmotic hyper- 

 tension (Bataillon) ? Experiments point to relative impermeability and 

 paralyzing auto-intoxication. 



Although hybridization experiments have proved that the spermato- 

 zoon may be the vehicle of generic, specific or varietal peculiarities, 

 Brachet argues that the spermatozoon is but an agent in division, and 

 has not the potentialities necessary for the formation of an organism. 

 H^e makes a good deal of the subsequently normal development of the 

 embryos of polyspermic frog-ova, and of Godlewski's fertilization of a 



