470 THE DOGFISH CHAP. 



disc. When segmentation of the oosperm takes place 

 it affects the protoplasmic part alone, the inactive yolk 

 taking no part in the process (compare crayfish, p. 383). 

 The polyplast stage consequently consists of a little 

 mass of cells, the blastoderm (Fig. 128), at one pole of 

 an undivided sphere of yolk. The cells of the blastoderm 

 become differentiated into the three embryonic layers 

 ectoderm, mesoderm, and endoderm. At the same 

 time the blastoderm extends in a peripheral direction 

 so as gradually to cover the yolk, and its middle part 

 becomes raised up into a ridge-like thickening, which 

 is moulded, step by step, into the form of the embryo 

 fish. The head, trunk, and tail acquire distinctness, 

 and become more and more completely separated off 



subsequently give rise to endoderm ana mesoaerm. 



sg. segmentation cavity ; below the blastoderm is the unsegmented yolk containing 

 scattered nuclei (n). (From Balfour's Embryology.) 



from the bulk of the egg, the latter taking the form of a 

 yolk-sac (Fig. 129, A, yk. s) attached by a narrow stalk 

 to the ventral surface of the embryo and supplied with 

 blood-vessels (see p. 599). 



In this condition the various parts of the adult fish 

 can be recognised, but the proportions are different and 

 the head presents several peculiarities. The gill-fila- 

 ments (br.f) are so long as to project through the external 

 branchial apertures and spiracles in the form of long 

 threads abundantly supplied with blood-vessels, and 



