Introduction to Animal 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 cytodes 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 embryo 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 aplanula),f an outer serous 

 or epiblastic, and an inner mucous or hypoblastic, 

 within which forms a cavity (the cleavage cavity or 

 cavity of I?<w]. The embryo in meroblastic eggs is 

 supported on subgerminal processes. Between these 

 layers there forms a mesoblast of one or two laminae. 

 The after-stages diifer 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- 

 cing lines, and each stage narrows the circle of 

 mblances, or as I \m l>acr expresses it, development 



ice from the general to the special. 

 Animal forms can be grouped into certain categories 

 call s+ each consisting of individuals identical 



In Entoconcha, Mullcr describes this as persisting and becoming trans- 

 formed into the nuclei of the cleavage cells. 



t "W: orption at one pole forms a mouth, and becomes a 



.la. 



J It is impossible to define species to the satisfaction of all nat 

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

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



