lufroduction to A iiiiiml Morphology. 37 



and spot,* cleaves into two, then into four, then eight, 

 &;c., until it is resolved into a morula or mulberry- 

 like mass of cytodes. The C3'todes develop nuclei and 

 become cells. This process may involve the whole 

 yelk circumference, as in Mammalia, &:c., and the eggs 

 are called holoblastic, the germ and yelk elements 

 being combined, or only a part, as in Birds, and the 

 rest is gradually absorbed as nutriment into the 

 cleaved part (meroblastic ova), the germ and yelk 

 being separate. The sphere of embrj^o cells produced 

 by the segmentation of the yelk is called the blasto- 

 derm, and in all animals above Protozoa this divides 

 into two layers (becoming a planula),t an outer serous 

 or epiblastic, and an inner mucous or hypoblastic, 

 within which forms a cavity (the cleavage cavity or 

 cavity oi Baer). The embryo in meroblastic eggs is 

 supported on subgerminal processes. BetAveen these 

 layers there forms a mesoblast of one or two laminae. 

 The after-stages differ in the different classes, and 

 will be considered under each head. Thus all animals 

 in their earliest embryonic stages are identical ; but 

 the process of differentiation takes place along di- 

 verging lines, and each stage narrows the circle of 

 resemblances, or as K^?;^ ^^z^:/' expresses it, development 

 takes place from the general to the special. 



Animal forms can be grouped into certain categories 

 called spccics,X each consisting of individuals identical 



* In Entcconcha, Mi'dler describes tliis as persisting and becoming trans- 

 formed into the nuclei of the cleavage cells. 



t "WTiich by absorption at one pole forms a mouth, and becomes a 

 gastrula. 



X It is impossible to define species to the satisfaction of all naturahsts, 

 as the conceptions in their minds are not the same, and species, genus, &c., 

 must be looked on as more or less arbitrary (ideal conceptions). 



