14 



Describe the different Kinds of Segmentation ( Cleavage], and 

 ' give Examples of the Animals in which each occurs. 



1. Eggs with little yolk divide completely and equally 

 (holoblastic and equal). This is seen in some Sponges, 

 Hydra, Echinoderms, certain Molluscs, Amphioxus (nearly 

 equal), and most Mammals (e.g., Lepus). 



2. Eggs with a considerable quantity of yolk at one 

 pole divide completely but unequally (holoblastic but 

 unequal). This is seen in some Sponges, in Lumbricus 

 and other Worms, in Molluscs, in Dipnoi, and in Rana. 



3. Eggs with a great quantity of yolk, with the small 

 formative disc at one pole, divide partially and discoidally 

 (meroblastic and discoidal). This is seen in Cuttlefishes, 

 in Elasmobranch fishes (e.g., Eaia and Scyllium\ in Birds 

 (e.g., Columba) and in Monotreme Mammals. 



4. Eggs with a large quantity of yolk, accumulated in 

 the centre, divide partially and superficially (meroblastic 

 and peripheral). This is seen in Arthropods (e.g., Astacus). 



What is a Blastula ? 



A blastula is a hollow developmental stage which has 

 one layer of cells. One hemisphere (the animal pole) 

 consists of small cells, and the other (the vegetal hemi- 

 sphere) has much larger ones more or less yolk-laden. 

 The cavity is the blastocoele or segmentation cavity. The 

 blastula is the first stage after cleavage or segmentation. 



Describe a Morula. 



A blastula which is practically a solid mass of cells and 

 in which the segmentation cavity is slight or absent. 



How is the Gastrula formed ? 



From the blastula by intucking (invagination) of one 

 hemisphere. The segmentation cavity becomes obliterated 

 and a new cavity (the archenteron or future food canal) 

 is -formed, its opening being the blastopore. 



