THE EGG, FERTILIZATION, MATURATION, AND CLEAVAGE 31 



beginning of cleavage. How this activation of the activation center is 

 brought about and whether it is caused by the entrance of the sperm, the 

 fusion of the pronuclei, or some other factor are unknown. 



CLEAVAGE IN EGGS OF THE EUTRACHEATA 



Although the eggs of most arthropods undergo superficial cleavage, 

 there are a number of exceptions. The following groups ma}^ be 

 recognized. 



1. Eggs with a Purely Superficial Cleavage. — This type, which is the 

 usual form, has already been described in the account of the development 

 of the generalized insect in Chap. II. The fusion nucleus divides into 

 daughter cells which by repeated division and migration to the periphery 

 give rise to a layer of cells or primary epithelium commonly known as the 

 "blastoderm." This tj^pe occurs with eggs rich in yolk, but it also occurs 

 exceptionally in certain cases where there is a deficiency in yolk as in 

 the eggs of aphids, some Platygastridae, and Hemimerus. 



2. Eggs with a Combination Cleavage — First Total, Later Super- 

 ficial. — -Combination cleavage that is total at first and later becomes 

 superficial has been observed in the group Collembola, and even among 

 them Anurophonis and Tetrodontophora undergo a purely superficial 

 cleavage. The combination type is well exemplified in Isotoma cinerea 

 described in Chap. XIII in which the entire egg mass divides until it 

 forms the 32- or 64-cell stage, each nucleus surrounded by a protoplasmic 

 envelope which in turn Ues in the center of a yolk spherule (Fig. 65). 

 The nucleated cell-like bodies now migrate, first to the surface of the 

 yolk spherules and then to the periphery of the egg, leaving the yolk 

 spherules in the center (Fig. 67), thus forming the blastula. The early 

 development of Anurida maritima as described by Clay pole (1898) does 

 not essentially differ. In the genus Tomocerus (Macrotoma) the develop- 

 ment starts with the division of the nucleus, but it is not until the third 

 or fourth division that the yolk also begins to divide. At the 32-cell 

 stage the yolk-cell walls disappear, and the yolk nuclei pass to the 

 periphery of the egg, where through tangential division of the nuclei the 

 blastoderm is completed. In the forms mentioned cleavage is equal; but 

 in Achorutes armatus, according to Uzel, an unequal cleavage takes place 

 with the formation of micro- and macromeres. A possible case of com- 

 bination cleavage has been briefly described by Strindberg for an ant of 

 the genus Azteca. 



3. Eggs with Purely Total Cleavage.^ — Purel}^ total cleavage occurs 

 in the eggs of several parasitic Hymenoptera and is associated in most 

 cases with polyembryony. As the process of cleavage in these forms is 

 complicated and is described in Part II in the section dealing with the 

 Hymenoptera, it will not be given here. Suffice to say it occurs among 



