8 EMBRYOLOGY OF INSECTS AND MYRIAPODS 



is unequal, the cells at the animal pole are small with but little yolk, and 

 the cells at the vegetative pole are larger and rich in yolk. 



Meroblastic eggs also are of two kinds: those in which cleavage is 

 discoidal and restricted to a small disk at the animal pole, whereas the 

 great mass of yolk at the vegetative pole does not divide, as with birds 

 and reptiles; and those in which the cleavage nucleus is a cytoplasmic 

 area located in the egg center surrounded by yolk, as with the arthropods. 

 In the second case a protoplasmic layer, the periplasm, covers the surface 

 and is connected with the central mass by a fine plasmic reticulum; the 

 cleavage nuclei as soon as they form, migrate to the surface to form a 

 superficial layer. 



Animals having holoblastic cleavage are the porifera, most coelenter- 

 ates, echinoderms, worms, mollusks except cephalopods, ascidians, 

 Amphioxus, amphibians, and the mammals except monotremes. Ani- 

 mals having meroblastic cleavage are the cephalopods, the bony fishes, 

 reptiles, birds, monotremes, arthropods, and a few coelentei'ates, of 

 which the last two have superficial cleavage, the others discoidal. 



Based on the amount of yolk present, eggs have also been divided 

 into alecithal, telolecithal, and centrolecithal. Alecithal eggs are those 

 in which the small amount of yolk is uniformly distributed throughout 

 the egg and in which cleavage is equal and holoblastic, as with the eggs 

 of echinoderms. Telolecithal eggs have considerable yolk and may be 

 either holoblastic, in which the entire mass undergoes unequal cleavage 

 as in the frog's egg, or meroblastic, in which only a disk at the animal 

 pole undergoes cleavage, as in the eggs of reptiles and fish. Centrolecithal 

 eggs, which are restricted to arthropods (exclusive of scorpions) and a 

 few coelenterates, are meroblastic, rich in yolk, and have superficial 

 cleavage. It is evident from what has been said that the type of cleavage 

 has no great significance so far as the systematic grouping of animals is 

 concerned, save that centrolecithal eggs with few exceptions are restricted 

 to the Arthropoda. 



References 



Doncaster (1920), Hegner (1914a), Hertwig (1912), Korschelt and Heider (1936), 

 Richards (1931), Schroder (Depdolla, 1928), Sharp (1934). 



