10 THE ANATOMY OF VERTEBKATED ANIMALS. 



while the egg reinains in the interior of the body of the 

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

 vertebrates which are termed ovoviviparous. Or, the young 

 may receive nourishment from its viviparoiis parent, before 

 birth, by the close apposition of certain vascular appendages 

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

 development. 



The vasctdar appendages in question constitute the chief 

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

 from the umbilical vesicle (as in Musfeliis 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 ordinarj' 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-masses, of 

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

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

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

 come very small, and are called embryo-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 

 iiniQr, the so-called mucous stratum [hypoblast), which gives 

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

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

 the cerebro-spinal nervous centres are evolved. Between 

 these appears the intermediate stratutn (mesoblast), which 

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

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



