672 EMBRYOLOGY : INVERTEBRATES 



save in the segment of the antenna, where they become the end- 

 sacs of the green glands. The later development of these animals 

 cannot be followed here, but it may be stated that it is quite 

 unlike that of the Vertebrata. 



tuc 



7X10 



Fig. 526. — Three stages in the segmentation of the egg of the crayfish. — From 



Parker and Haswell. 



nu,, Nuclei ; y.p., " yolk-pyramids." 



In the polyclad Turbellaria, the Polychseta and the Mollusca 

 there is a peculiar type of holoblastic division of the egg known 

 as spiral cleavage, which is quite different from the regular 

 radial cleavage which we have seen in the lancelet and frog. 



Pcdnr. ih^aJbcL '^^^ 



Fig. 527. — Part of a longitudinal section through the egg of a crayfish after the 

 enteron has been established and the blastopore has closed. — From Parker 

 and Haswell, after Reichenbach. 



ccl., Ectoderm; end., endoderm ; mt., enteron; mes., mesoblast ; palm., proctodaeum (for hind gut); 

 atdm., stomodaeum (for fore gut) ; th.abd., rudiment from which abdomen and part of thorax arise; 

 yk., yolk, lying at this stage in what is morphologically the blastoccele. 



The first four cells are equal in size and lie in the same plane, 

 but slightly overlap one another (Fig. 528). They can be dis- 

 tinguished as A, B, C, and D, and while A and C are lateral, 

 B will form the ventral part of the embryo and D the dorsal part. 

 These cells, or macromeres as they come to be called, divide 

 three times to bud off three quartets of smaller cells or micro- 

 meres, called a^, b^, c^, d^, ag, etc., according to the macromere 

 from which they originate and the order of their formation. 



