280 CLEAVAGE (SEGMENTATION) AND BLASTULATION 



e) Morula stage 



f) Early blastocyst 



3) Types of mammalian blastocysts (blastulae) 



b. Holoblastic cleavage of the transitional or intermediate type 



1) Anihy stoma maciilutuin (punctatum) 



2 ) Lepidosiren paradoxa 

 3 ) Nee turns maculosus 



4) Acipenser stiirio 

 5 ) A mia calva 



6) Lepisosteus (Lepidosteus) osseus 



7) Gyinnophionan amphibia 

 3. Meroblastic cleavage 



a. Egg of the common fowl 



1 ) Early cleavages 



2) Formation of the periblast tissue 



3) Morphological characteristics of the primary blastula 



4) Polyspermy and fate of the accessory sperm nuclei 



b. Elasmobranch fishes 



1 ) Cleavage and formation of the early blastula 



2) Problem of the periblast tissue in elasmobranch fishes 



c. Teleost fishes 



1) Cleavage and early blastula formation 



2) Origin of the periblast tissue in teleost fishes 



d. Prototherian Mammalia 



e. Cleavage in the California hagfish, Polistotrema (Bdellostoma) stouti 



C. What is the force which causes the blastomeres to adhere together during early 

 cleavage? 



D. Progressive cytoplasmic inequality and nuclear equality of the cleavage blastomeres 



1. Cytoplasmic inequality of the early blastomeres 



2. Nuclear equality of the early blastomeres 



E. Quantitative and qualitative cleavages and their influence upon later development 



A. General Considerations 



1. Definitions 



The period of cleavage (segmentation) immediately follows normal fertili- 

 zation or any other means which activates the egg to develop. It consists of 

 a division of the entire egg or a part of the egg into smaller and smaller cellular 

 entities. In some species, however, both chordate and non-chordate, the early 

 cleavage stages consist of nuclear divisions alone, to be followed later by the 

 formation of actual cell boundaries (fig. 62). The cells which are formed 

 during cleavage are called blastomeres. 



As cleavage of the egg continues, the blastular stage ultimately is reached. 

 The blastula contains a cavity or blastocoel together with an associated layer 

 or mass of cells, the blastoderm. The blastula represents the culmination and 

 end result of the processes at work during the cleavage period. Certain aspects 

 and problems concerned with blastulation are considered separately in the 

 following chapter. However, the general features of blastular formation are 

 described here along with the cleavage phenomena. 



