284 



ZOOLOGY. 



the nuclei assume a superficial position where they become 

 surrounded by protoplasm, and form the blastoderm (Fig. n, 

 D, j). This is described as partial and peripheral segmenta- 

 tion. On the side of the egg where the embryo is to lie, a 

 thickening called the ventral plate is formed. From this area 

 of the blastoderm there arises, by specialization, by insinking, 

 and by multiplication of the cells, the three-layered condition. 

 The presence of yolk so obscures and complicates the process 

 that the student must be referred to more comprehensive books 

 for even an outline of it. 



323. The Later Development may be either direct or in- 

 direct. That is to say, the young when hatched may be the 

 adult in miniature, possessing its form and habits, or may 

 have a very different form and assume the adult condition by 

 one or more metamorphoses. The differences between the 

 larval and adult conditions may be slight or very great. To 

 effect the change from larva to adult a series of moultings of 



FIG. 126. 



FIG. 126. The Zoea of Cancer irroratus. From Verrill. x 15. 



Questions on the figure. Compare with the larva of lobster (Fig. 129) 

 and with the Megalops (Fig. 127), and note likenesses and differences. 



