34 THE DEVELOPMENT OF 



ducing the structure of the parent. In the Nauplius there 

 is little that is non-essential in an Arthropod, and we must 

 regard it as an embryo of the type just mentioned. It has 

 an alimentary tract, traversing an unsegmented body, a 

 median eye, and three pairs of appendages, the first sen- 

 sory, the other two pairs at once natatory and fitted for the 

 comminution of food. The most salient features in subse- 

 quent growth are the elongation and segmentation of the 

 body and the addition of more appendages posteriorly. It 

 is noticeable that with increase in food yolk there is a 

 marked tendency towards obliteration of the nauplius stage 

 in the Crustacea ; in fact, it may really be questioned whether 

 the so-called nauplius stage in the decapods and tetrade- 

 capods exists only because of inheritance from such an an- 

 cestor or whether it be but a necessary sequence of meta- 

 merism, for in any segmented animal the anterior are the 

 first parts to appear and hence in all there must be a time 

 in development when but three segments exist. Of course 

 it should not be forgotten that a free-swimming uauplius 

 stage occurs in Lucifer and Penoeus, and that Lucifer, at 

 least, has an egg in which food yolk is not abundant. 



SUMMARY. 



1. The arthropod egg is not to be regarded as cen- 

 trolecithal and having a superficial segmentation but as 

 having a central segmentation, the blastoderm being formed 

 by migration of the resulting cells to the surface. 



2. The primitive groove in the Arthropods is a modified 

 blastopore, and the absence of invagiuated entoderm in 

 some forms is to be explained by Cope's and Hyatt's the- 

 ory of acceleration and retardation. 



3. In Crangon the anus occupies the position of the 

 blastopore. 



4. In Crangon and many other Crustacea the young ger- 

 minal area is actually larger than the much older embryo. 



