54 KINGS LEY. [Vol. VII. 



(p. 50). Patten has incidentally described some of the early 

 stages of Limulus ('90). Packard, Dohrn, Lockwood, et al. have 

 described the later stages, and the foregoing brief resu7ne calls 

 for no comparisons with their results. (See Postscript.) 



B. With Other Arthropods. — Three types of segmentation 

 of the ^gg may be recognized in the Arthropods. 



In the first, examples of which are furnished by the lower 

 Crustacea, Lucifer, (i") Palaemon (Bobretzky), Phronima, Chelifer, 

 Gammaj'ns iocusta (Van Beneden and Bessels), Pycnogonids 

 (Morgan), etc., the &gg undergoes a regular or irregular total 

 segmentation (holoblastic). 



In the second the egg consists of a central nucleus and 

 protoplasm with peripheral yolk. The central protoplasm seg- 

 ments, but until several or many blastomeres result, the yolk 

 remains undivided. This is the type usually called cen- 

 trolecithal, or endolecithal (Claus) and superficial. I have 

 already pointed out with some detail ('86, pp. 11 2-1 38) that 

 these terms are misleading, and would substitute ectolecitJial 

 therefor. ' Superficial segmentation ' as usually described is 

 characteristic only of late stages of ectolecithal or of meroblastic 

 eggs. In these ectolecithal eggs two secondary modifications 

 are noticeable. In the one the yolk is extracellular ; it lies 

 between the cells formed by the dividing protoplasm and 

 nuclei, as in Phryganids (Patten, '85), Crangon (Kingsley, 

 '86), and Julus (Heathcote, '86). In the other the yolk itself 

 becomes divided, forming balls (true yolk cells), in the centre 

 of each of which the nucleus and protoplasm occur (examples, 

 most Hexapods).^ Of these the second is structurally, if not 

 phylogenetically, nearest to the meroblastic type. 



In the third or meroblastic type the segmentation is, strictly 

 speaking, superficial, and is at first confined to one side of the 

 ^gg. Instances are less common among the Arthropods than 

 of the other two, although several have been described; e.g. 

 Scorpio (Metschnikoff, '71 ; Laurie, '90), Mysis (Van Beneden), 

 Oniscus^ (Bobretzky). 



1 Mereschowski ('82) has described what he regards as a fourth type, occurring in 

 Callianassa mediterranea. It is plainly closely related to the second modification 



just mentioned. 



2 According to Reinhard's brief note ('87) it would appear as if in Porcellio 

 the segmentation was of the ectolecithal type, and that the meroblastic conditions 



