CHAPTER XIV 



ELEMENTS OF VERTEBRATE EMBRYOLOGY 



The Vertebrate like other animals begins its individual existence as a 

 unicellular zygote formed by the coming together of two gametes, and 

 this becomes gradually converted into the multicellular adult by a com- 

 plicated series of developmental processes. These should be studied first 

 in the case of Amphioxus, for it is in this animal that at least the earlier 

 phases of development are to be found taking place in the simplest and 

 least modified fashion. 



The macrogamete or egg of Amphioxus is a relatively minute spherical 

 cell about -i mm. in diameter which is shed into the atrial cavity and 

 passes thence through the atriopore to the exterior where syngamy takes 

 place as a rule at once, myriads of microgametes having become dis- 

 seminated through the sea-water synchronously with the emission of the 

 eggs by the females. The act of syngamy is followed by segmentation. 

 The zygote nucleus undergoes mitosis and a deep valley or furrow encircles 

 the egg, gradually deepening and dividing the egg into two halves each 

 of which rounds itself off into spherical form (Fig. 187, B and C). Careful 

 study has shown that these two first blastomeres represent the right and 

 left halves of the new individual. 



Each blastomere now becomes subdivided by a precisely similar 

 furrow or valley in a plane at right angles to that of the first — the result 

 being that there are now four blastomeres (Fig. 187, D). These two first 

 furrows are meridional : in other words they resemble in position the 

 meridians of a terrestrial globe, passing through the upper or apical pole 

 of the egg. There now appears a latitudinal furrow — corresponding in 

 position to a parallel of latitude a little above the equator of a terrestrial 

 globe — so that the egg consists now of four smaller blastomeres towards the 

 apical pole, and four rather larger towards the opposite pole (Fig. 187, E). 

 Other furrows make their appearance on the surface of the blastomeres 

 and gradually deepening divide them into two, and as this process goes 



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