116 LECTURES ON BIOLOGY 



acquire such new adaptations to special conditions of life. It 

 behoves us, therefore, to proceed with the utmost caution in 

 drawing phylogenetic conclusions on the basis of embryological 

 evidence. It would, for instance, be erroneous to assume, 

 because in the larva of the Libellulse the second pair of maxillae 

 (labium) is modified into a pair of long nippers for deftly seizing 

 the prey, that the dragon-flies had descended from a similarly 

 constructed ancestor. 



All multicellular organisms consist at the starting point 

 of their development of a single cell, the ovum, or, more 

 accurately, the fertilized egg-cell. For with the exception of 

 a few cases it is necessary that two cells, the male spermatozoon 

 and the female ovum, unite in order to make the origin of a 



FIG. 31. OVA OF DIFFERENT ANIMALS. 



(1) Egg of a fowl ; (2) egg of a sea-cucumber (Holothuria tubulosa) ; (3) egg of 

 fresh- water polyp Hydra. (I, Less than natural size ; 2 and 3, greatly magnified.) 



new multicellular organism possible. The process during which 

 unification takes place is called fertilization ; the result is the 

 fertilized egg-cell. We shall have an opportunity later on to 

 deal with this subject in greater detail. 



The similarity, in their minute structure, of ova of even the 

 most distant species is remarkably great. Indeed, the resem- 

 blance is often so complete that in many cases even an expert 

 finds it difficult to distinguish between them. This difference in 

 appearance, however, is due to secondary factors conditioned 

 apparently by specific conditions of life ; nevertheless the general 

 structure is always essentially alike. The ovum of whichever 

 animal we may examine, we shall always find it to be a simple 

 normal cell, consisting of the cell-body protoplasm and the cell- 



