Till 



ARTHROPODA 



195 



nucleus divides three times successively, and at each division gives 

 ! off a daughter which migrates to the surface and segregates round 

 itself a blastoderm cell, the pre-existing blastoderm cells also dividing 

 each time. In this way an investment of the yolk by blastoderm 

 cells is effected. At the fourth cleavage the mother nucleus gives 

 rise to a primary mesoderm cell in front and then conies to the surface 

 itself as the first endoderm cell. Subsequently both mesoderm and 

 endoderm cells divide into right and left halves, and the endoderm 

 cells withdraw from the surface. This inwardly directed cytotaxis 

 is the process of gastrulation. The cells bordering the blastopore at 

 the anterior end bud off cells which sink inwards ; these may be termed 

 mesectoderm, and they are perhaps equivalent to the secondary 

 mesoderm of Astacus. A me so dermal band is formed which extends 

 forwards and upwards, and along its course three outwardly directed 

 transverse grooves delimit the three pairs of appendages of the 



mes.b 



Fro. 143. Four sagittal sectioiis through the developing eggs of Lepas anatifera in 

 different stages of development. (After Bigelow. ) 



A, 15-cell stage, formation of mesoderm. B, 16-cell stage, mesoderm formed. C, 62-cell stage, 

 formation of mesectoderm. D, 250-cell stage, formation of mesodermic baud. Up, blastopore ; end, endo- 

 dermal nucleus ; mes, mesoderm ; mes.ect, mesectoderm ; ines.b, jnesodermic band ; j&, second polar body. 



Nauplius. The dorsal extension of these appendages is in reality the 

 same phenomenon as their outward direction in Astacus; and the 

 apparent difference in direction is due to the smaller quantity of yolk 

 in the egg of Lepas. 



The development of Copepoda, to judge from the somewhat con- 

 flicting accounts which we possess, seems to be essentially similar to 

 that of Lepas. 



A renewed study by modern methods of the development of a 

 primitive form like Branchipus would throw a flood of light on the 

 development of Crustacea generally, and perhaps enable us to under- 

 stand the conflicting accounts given of the development of Copepoda. 



LARVAL HISTORY THE NAUPLIUS J 



With regard to the larval history, we may take as type the 

 development of the common fresh-water Copepod Cyclops, of which 

 various species are found in fresh water all over the world. If females 

 of Cyclops carrying egg-sacs are isolated and kept in small shallow 



