50 PRINCIPLES OF EMBRYOLOGY 



than that of unfertilised eggs, and it was thought that fertiHsation brought 

 about a great increase in respiration. However, more recent work has 

 shown that the difference he described is due rather to a decrease in 

 oxygen uptake by the stale unfertiHsed eggs than to an increase in the 

 fertilised ones. Rothschild (1951^), in a recent discussion of the metabohc 

 changes produced by fertilisation, is very cautious about the extent of 

 our knowledge on the subject. He suggests that the production of an 

 acid of unknown nature is one of the most certain and striking phenomena, 

 and hsts a number of other changes, such as a reduction in glycogen 

 content and a fall in respiratory quotient, without feeling justified in 

 deciding which if any of these are of major importance. 



(2) The union of the nuclei 



The union of the two haploid nuclei of the egg and sperm is, from the 

 long-term point of view, the most important phase of fertilisation. It is an 

 essential part of the system of reproducing diploid aduks by means of 

 haploid gametes which has proved itself most efficient as an evolutionary 

 mechanism and has therefore been perpetuated in the vast majority of 

 animals and plants. The genetical and evolutionary aspects of the pheno- 

 menon fall outside our immediate field of interest. We must, however, 

 give some account of the actual process by which the two nuclei come 

 together. 



The essential parts of the sperm in this connection are the head, which 

 contains the nucleus, and the middle-piece, which contains a centrosome 

 or spindle-body. The tail is primarily a locomotor organ, and its functions 

 are finished when the sperm head becomes attached to the surface of the 

 egg. It is often discarded at this point, only the head and middle-piece 

 penetrating; and in those cases in which the tail goes in too (e.g. in mam- 

 mals), it soon degenerates and plays no known part in later events. 



The actual coming together of the two nuclei is simple, though myster- 

 ious enough if one tries to imagine how it works. The sperm nucleus 

 after penetration always moves off towards the egg nucleus, while the 

 latter sometimes moves to meet it; what moves them, and how the 

 movements take the correct directions, are quite unknown. The sperm 

 head (or as one may now call it the male nucleus, often called the male 

 pronucleus), is accompanied by the middle-piece or centrosome. This 

 body has a strong tendency to cause the cytoplasm around it to form an 

 'aster', which is a spherical aggregation of radiating fibres. If often starts 

 to do so soon after getting inside the egg; but it soon divides, and an 

 aster begins to form round each of the daughter centromeres. Where 

 these two asters come together, a still more strongly fibrous body is 



