REPRODUCTION AND DEVELOPMENT IN CHORDATES 



of the blastoderm to form the yolk sac, in which blood vessels soon appear. The amnion has 

 been formed in F and G but it is difficult to distinguish; in F the small sac at the posterior 

 end is the allantois, which is also seen in G as the smaller of the two appended sacs. H and 

 /, later embrvos removed from their shells, with the amnion and allantois removed; only the 

 volk stalk remains in H, and the yolk sac is shown in /. /, the head of an embryo removed 

 and viewed from the front, showing mouth, sense organs, and remains of the gill furrows. (Re- 

 drawn from M. Duval, Atlas d'embryologie, 1889.) 



persist as rudimentary, non-functional organs, vestiges of the indifferent 

 period of development. 



Differentiation of the Germ Cells 



The somatic cells are divisible into groups on the basis of characteristic 

 features of the structure of their cytosomes (pp. 58-69). These cells 

 are specialized, or differentiated, to perform certain functions related to the 

 maintenance of the life of the individual. There are other cells, the germ 

 cells, which become differentiated in connection with the capacity of repro- 

 duction, as a result of which continuity of the species is assured. In other 

 words, new individuals arise from the germ cells of previously existing 

 individuals. 



It has been stated that cells arise by division of previously existing cells, 

 and this statement is true of the germ cells as well as of the somatic cells. The 

 primordial germ cells arise, of course, from the same ancestor that gives rise to 



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