276 



ANIMAL PHYSIOLOGY. 



eggs, adds or withholds a little of the contents of the sac; when 

 this is withheld the egg produces a male; but when it is 

 added a female is produced, which, according as it is fed, 

 becomes a worker or a queen. 



In vertebrata, sexual reproduction is the only kind which 

 occurs. Yet in the development of the embryo there is a 

 set of phenomena quite homologous with alternate genera- 

 tion; for the whole ovum is not converted into an embryo, 



but only a part of it, and, there- 

 fore, the embryo may be legi- 

 timately considered as a bud 

 from the ovum; in which case 

 the only difference between ver- 

 tebrate reproduction and that 

 of a medusa is, that in the 

 medusa many buds are derived 

 from an ovum, and in the ver- 

 tebrata there is only one.* As 

 an abnormal variation, the single 

 vertebrate embryo may divide 

 more or less completely; and 

 this is the origin of all double 

 such as two- 

 headed calves, four-legged hens, 

 the " Siamese twins," and the negress sisters exhibited as the 

 " two-headed nightingale." The proof of this is found in the 

 fact that embryos in early stages of development have been 

 seen thus partially divided, and in the law of double mon- 

 strosities that they are always united by corresponding parts. 

 201. Akin to the power of reproducing the whole individual 

 is the power of reproducing lost parts; and the law may be laid 

 down that the less advanced the development of the species 

 or the individual the greater the power of such reproduction. 

 So great is this power in some invertebrate animals that they 

 may be multiplied by artificial division, each moiety retaining 



* Viewing the vertebrate embryo as a bud from the ovum, a com- 

 parison by no means vague may be drawn between its development 

 and that of a tooth. In both instances there is an elevation which 

 becomes surrounded by a fossa, afterwards converted into a shut sac, 

 and finally the shut sac is burst. 



Fig. 139. DOUBLE-HEADED EM- 

 BRYOS. A, Chick (in my pos- monstrosities, 

 session). B, Perch (Von Baer). 



