10 THE ANATOMY OF VERTEBRATED ANIMALS. 



while the egg remains in the interior of the body of the 

 parent, but quite free and unconnected with it ; as in those 

 vertebrates which are termed ovovivi2yarous. Or, the young 

 may receive nourishment from its vivijyarous parent, before 

 birth, by tlie close apposition of certain vascular appendages 

 of its body to the walls of the cavity in which it undergoes its 

 development. 



The vascular appendages in question constitute the chief 

 part of what is called the placerita, and may be developed 

 from the umbilical vesicle (as in Mustelus among Sharks), or 

 from the allantois and chorion (as in most mammals). At 

 birth, they may be either simply detached from the substance 

 of the parental organism, or a part of the latter may be 

 thrown off along with them and replaced by a new growth. 

 In the highest vertebrates, the dependence of the young upon 

 the parent for nutrition does not cease even at birth ; but 

 certain cutaneous glands secrete a fluid called milk^ upon 

 which the young is fed for a longer or shorter time. 



When development takes place outside the body, it may 

 be independent of parental aid, as in ordinary fishes ; but, 

 among some reptiles and in most birds, the parent supplies 

 the amount of heat, in excess of the ordinary temperature of 

 the air, which is required, from its own body, by the process 

 of incubation. 



The first step in the development of the embryo is the 

 division of the vitelline substance into cleavage-inasses^ of 

 which there are at first two, then four, then eight, and so on. 

 The germinal vesicle is no longer seen, but each cleavage- 

 mass contains a nucleus. The cleavagfe-masses eventually be- 

 come very small, and are called emhryO'Cells, as the body of 

 the embryo is built up out of them. The process of yelk- 

 division may be either complete or partial. In the former 

 case, it, from the first, affects the whole yelk ; in the latter, 

 it commences in part of the yelk, and gradually extends to 

 the rest. The blastoderm^ or embryogenic tissue in which it 

 results, very early exhibits two distinguishable strata — an 

 inner, the so-called mucous stratum (/iT/poblast) , which gives 

 rise to the epithelium of the alimentary tract ; and an outer, 

 the serous stratum (ejnblast), from which the epidermis and 

 the cerebro-spinal nervous centres are evolv^ed. Between 

 these appears the intermediate stratum (mesoblast)^ which 

 gives rise to all the structures (save the brain and spinal mar- 

 row) which, in the adult, are included between the epidermis 



