THE ARGUMENTS FROM EMBRYOLOGY. 36i) 



occasionally great. Tlie hypothesis of evolution accounts for 

 these : it does more — ^it implies the necessity of them. 



§ 130. The substitutions of organs and the suppressions 

 of organs, are among those secondary embryological phe- 

 nomena which harmonize with the belief in evolution 

 but cannot be reconciled with any other belief. There are 

 cases where, during its earlier stages of development, an 

 embryo possesses organs that afterwards dwindle away, as 

 there arise other organs to discharge the same functions. 

 And there are cases where organs make their appearance, 

 grow to certain points, have no functions to discharge, and 

 disappear by absorption. 



We have a remarkable instance of this substitution in the 

 successive temporary appKances for aerating the blood, 

 which the mammalian embryo exhibits. During the first 

 phase of its developm.ent, the mammalian embryo circulates 

 its blood through a system of vessels distributed over what 

 is called the area, vnsoidosa — a system of vessels homologous 

 with one which, among fishes, serves for aerating the blood 

 im-til the perm.anent respiratory organs come into play. 

 After a time, there buds out from the mammalian embryo, a 

 vascular membrane called the allantois, homologous with 

 one which, in birds and reptiles, replaces the first as a 

 breathing apparatus. But while in the higher oviparous 

 vertebrates, the allantois serves the purpose of a lung during 

 the rest of embryonic life, it does not do so in the mamma- 

 lian embryo. In implacental mammals, it aborts, having no 

 function to discharge ; and in the higher mammals, it 

 becomes " placentiferous, and serves as the means of inter- 

 communication between the parent and the offspring " — be- 

 comes an organ of nutrition more than of respiration. Now 

 since the first system of external blood-vessels, not being in 

 contact with a directly-oxygenated medium, cannot be very 

 eerviceable to the mammalian embryo as a lung ; and since 



