82 



CYTOLOGY 



CHAP. 



thrown out and the middle pieces break up, as in /I. megalocephala, except 

 that only two instead of about thirty small chromosomes are produced 

 from each original one. 



In many animals the germ-track is marked out from the undivided 

 egg onwards by characteristics of the cytoplasm instead of the nucleus. 

 The fresh-water crustacean Cyclops furnishes an example. The germ- 

 track in this animal was first worked out by Hacker in C. viridis (1897 a), 

 his results being confirmed in all essentials by Amma in 191 1, who also 



Germ Cells 



Fig. 38. 



Scheme of the cleavage divisions in Ascaris megalocephala. (Boveri, Ergebnisse, 1904.} 

 The uppermost cell is the fertilized ovum. 

 Cell in which chromatin diminution has not taken place. 

 ]0.' Cell in which chromatin diminution takes place. 

 O Cell with diminished chromatin. 



discovered the same process in several different species of Cyclops and 

 in the allied genera Diaptomus and Canthocamptus. The account given 

 by the latter author for Cyclops fuscus will serve for an example (Fig. 39). 

 In the prophase of the first cleavage division one attraction sphere 

 is distinguished from the other by a group of granules which surrounds 

 it, these being completely absent from the other sphere. Consequently, 

 at ceU division all the granules pass into one of the first two cells or 

 blastomeres and none into the other ; nor do they ever appear in the 

 descendants of the latter cell. In the case of the blastomere containing 

 the granules the process is repeated in the following mitosis. After 

 the first cell division is completed the granules become clumped together 



