108 



CRUSTACEA. 



The following four types of cleavage* may be distinguished among 

 the Crustacea : — 



Type I. Eggs with complete^and equal cleavage. This type 

 is of very rare occurrence among the Crustacea. It is, however, to 

 be found in the egg of Lucifer (Brooks, No. 43, Fig! 49), which is 

 very poor in yolk. In this egg, after cleavage has taken place in the 

 most regular manner, there is formed a coeloblastula consisting of 

 few cells (Fig. 49 B) and having a spacious central cleavage-cavity, 



a 



.--: 



\ 



B 



a 



Fig. 50.— Fertilisation and cleavage of the egg of BrancMpus (after A. Brauer). A, fertilisa- 

 tion stage. B and C, early stages showing total cleavage. D, older stage with superficial 

 cleavage, c, vitelline membrane ; d, secondary egg-shell ; /, cleavage-cavity ; pf, female 

 pronucleus ; pm, male pronucleus ; r, polar body. 



this stage giving rise to a very primitive invagination-gastrula (Fig. 

 49 C). All the cells at first appear to be similar in form and equally, 

 provided with spherules of yolk. At the commencement of the 

 process of invagination, however, one cell lying at the vegetative 1 

 pole (Fig. 49 B, c) is to be distinguished by the greater accumulation 



* It should be mentioned that J. Nusbaum (No. 39) similarly distinguishes 

 four types of cleavage among the Crustacea. His Types I. and II., however, doi 

 not agree with those here given. 



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