xlvi 



THE ANIMAL KINGDOM. 



as an elimination of the male portion from the egg, while, mutatis mutandis, the same 

 may be said of the remnants of the mother-cells from which the spermatozoa are 

 formed. 



Now the egg is ready for that union with the male element, or spermatozoa, which is 



called fertilization or impregnation. The process in most if not all forms is essentially 



as follows : One or more spei'matozoa enter the egg ; in some 



cases it has been found that, if more than one entered, the re- 



~ suit was a malformation, to be noticed below ; in other eggs, 



on the contrary, several spermatozoa are necessary for fertili- 



-/ zation. As soon as the head of the spermatozoon enters the 



FIG. VII. -Entrance of sperma- egg, it f orms a clear space known as the male pronucleus. 



tozoan into egg, and forma- Around this radial strisB appear, and it slowly travels toward 



tion of male pronucleus (m); rr i j 



/.female pronucleus; s.sper- the female pronucleus until the two unite. This compound 



matozoan. -^ ^ 



structure, thus foimed, is known as the segmentation nucleus. 

 Now begins the segmentation which was de- 

 scribed briefly on a preceding page, which results 

 in the conversion of the egg into a mass of cells, 

 and which need not be repeated here. One inter- 

 esting fact, however, may be mentioned. Hermann 

 Fol, in his studies on the development of the star- 

 fish, found that, if several spermatozoa obtained 

 entrance to the egg, a corresponding number of 

 segmentation nuclei were formed ; and although 

 development proceeded but a short distance, the 

 results of this abnonnal condition were visible 

 throughout. Each nucleus formed a centre of seg- 

 mentation, and when the time arrived for the form- 

 ation of a gastrula, the same influence was felt, and, 

 as shown in the adjacent cut, there were several 

 invaginations. These observations possess a high interest from a teratological point 

 of view, as they may afford an explanation of the formation of double monsters. 



Metamorphosis. 

 After the formation of the germ layers as described on pages ix. to xii., the develop- 

 ment of the various organs proceeds, for the details of which one should consult the 

 accounts of the different groups in the body of this work, and especially Balfour's clas- 

 sic Treatise on Comparative Embryology. Still we may consider here some of the 

 questions connected with metamorphosis. This term, which has been employed for 

 many years, is used to indicate the series of changes which an animal undergoes after 

 being born or after hatching from the egg. In some cases the changes are very slight, 

 the young leaving the egg in nearly the adult form, while in others, of which a famil- 

 iar instance is furnished by the butterfly, the modifications which are introduced be- 

 tween the egg and the mature condition are most startling. As other examples of 

 these complete metamorphoses, we would refer the reader to the jelly-fish, star-fish, sea- 

 urchins, worms, molluscs, insects, crustaceans, and batraohians, as described in the body 

 of this work. One of the most curious is that presented by the larval form known as 

 Actinotrocha, which converts itself into the mature worm Phoronis, by apparently 

 turning itself inside out. If the reader will compare different accounts ; and notice that 



Fig. VIII. — Abnormal gastrulation in an 

 echinodenn, the result of multiple im- 

 pregnation. 



