ON THE DEVELOPMENT OF PARASITIC COPEPODS. 65 



tiated from the somatic cells at the close of the fourth cleavage 

 or one generation earlier than in the parasitic copepods. 



Boveri ('92) in Ascaris, and Haecker ('97) in Cyclops traced 

 the " Keimbahn " from the first cleavage. In Ascaris the visible 

 difference between germ cell and somatic cell was in the chromo- 

 somes, in Cyclops in the cytoplasm. Early differentiation of the 

 germ cells has been noticed in a large number of animals, but 

 the causal factors in their differentiation are yet unknown. From 

 Boveri's account of Ascaris, it seems that the cells of the " Keim- 

 bahn " preserve all the characters of the fertilized egg, while the 

 somatic cells lose some characters. Yet the mature ova and 

 spermatozoa of most animals are possibly as highly differentiated 

 as any somatic cell. 



In the dichelestid the germ cells have the same origin as in 

 Pandarns simtatns but they differ in appearance. Fig. 57 shows 

 the primary germ cell beginning to be turned under the blasto- 

 poral rim. Fig. 58 shows a stage after the division of the germ 

 cell into two cells (of unequal size). If we followed Pedaschenko's 

 theory we might consider the large cell as female and the small 

 cell as male as it is always true that one is larger than the other. 

 The nuclei of the two germ cells lie in their ends that are nearest 

 the free border of the blastoderm (blastopore). These two cells 

 divide into four and the nuclei of two are larger than of the 

 other two, but the cell boundaries between them are extremely 

 difficult to make out. 



B. The Mesoblastic Rudiments of the Nanpliits Appendages 

 (Pandarus sinuatus} arise from cells turned under the rim of the 

 blastopore during epibole. When the cap of protoplasmic cells 

 has covered about one third the yolk (Fig. 38) a few cells are 

 turned under the rim at the extreme right and left, that is to 

 say at the edge of the disc shaped egg. These cells are the 

 mesoblastic elements of the first and second antennae and divide 

 on each side into two masses (Fig. 40, an 1 , ati 2 }. The time 

 of this division varies slightly, the elements being sometimes 

 widely separated before closure of the blastopore (Fig. 40) and 

 sometimes close together just after the closure of the blastopore 

 (Fig. 41, an 1 , atr). Just before closure of the blastopore, a 

 few cells are turned under its lip on each side (Fig. 40, uui] and 



