304 EMBRYOLOGY 



results from two successive cell divisions, and in one of these divisions 

 chromosomes do not divide as in normal cell division. 



To understand the elements of this reduction division, or meiosis, we 

 shall begin with the fertilization of an egg, which will ultimately give rise 

 to the germ cells in the adult. Imagine a sperm having two chromosomes 

 and an egg with the same number. Shortly after fertilization the chromosomes 

 of egg and sperm fuse and thus we have an egg with four chromosomes. Dur- 

 ing each cell division the chromosomes reduplicate and as a result all cells, 

 including the primitive germ cells in the adult, possess four chromosomes 

 (Fig. 194). The behavior of the primitive germ cells is shown diagrammati- 

 cally. From each oocyte a single mature egg results, whereas from each 

 spermatocyte four functional sperm develop. Thus in order to illustrate the 

 four chromosome combinations possible in th^ egg we must begin with four 

 oocytes in the ovary. 



In Figure 194 the paternal chromosomes are white and maternal chromo- 

 somes are black. Each oocyte undergoes two divisions, and each division is 

 unequal, with the result that a mature egg and three small cells, or polar 

 bodies, are formed. The polar bodies have no function, and thus we have 

 to consider only the four types of eggs. It will be seen from the diagram that 

 there is a random segregation of the chromosomes of the oocyte into the 

 mature egg. 



Similarly the spermatocyte undergoes two divisions, but these are equal 

 and four sperm result with four different chromosome combinations. 



Fertilization of the egg by the sperm is also a random union, with the 

 result that all possible combinations of chromosomes are found in the 

 fertilized egg or zygote. In Figure 194 nine different recombinations are 

 shown. Actually many more recombinations occur since an egg and sperm 

 may have as many as 100 chromosomes. The normal number of chromosomes 

 is restored by fertilization. Since the chromosomes are present in duplicate, 

 the genes are also present in duplicate in the fertilized egg. Duplicate genes 

 are not necessary for most of development, for rather normal development 

 may occur in parthenogenetic eggs containing only the egg nucleus. 



Although development is normal when only half the number of 

 chromosomes is present in a parthenogenetic egg, the addition of foreign 

 chromosomes to an egg produces profound effects in development. Thus, 

 the crossing of two common species of frogs results in a fertilized egg which 

 develops perfectly normally to the beginning of gastrulation but then stops 



