1 90 COMPARATIVE ANATOMY 



eventually burst, discharging their contents into the atrium, 

 whence they pass through the atriopore to the exterior. 

 Fertilisation is effected in the water after the discharge of 

 the ova. 



The development of Amphioxus is fully as interesting and 

 instructive as its adult anatomy, and the study of it may be 

 regarded as the indispensable introduction to the study of 

 vertebrate embryology. It is, however, beyond the scope of 

 this book to enter into it in any detail. The reader will find 

 a full account of the embryonic and larval development in Dr 

 Willey's excellent treatise on the subject,* and much may be 

 learned from almost any book on vertebrate embryology. 

 The following account will deal only with points of funda- 

 mental importance, and the full explanations attached to 

 figures 47-50 will spare the necessity of lengthy descrip- 

 tion. The first stage of development includes the segmen- 

 tation of the ovum and the formation of the gastrula. The 

 ovum is spherical, and sparsely furnished with food-yolk. 

 The first cleavage is vertical, and divides the ovum into 

 two equal halves. The second cleavage is also vertical, 

 at right angles to the first, and results in the formation of 

 four equal blastomeres. The third cleavage is nearly equatorial, 

 dividing each blastomere into a somewhat smaller upper cell 

 and a larger lower cell. The fourth cleavage divides all the 

 cells vertically, producing a sixteen-cell stage, with eight upper 

 and smaller and eight lower and larger cells surrounding a 

 central cavity widely open to the exterior above and below. 

 By continued meridional and transverse divisions the sixteen- 

 cell stage is converted into a hollow sphere or blastula, with 

 a central cavity (blastocoele) surrounded by a single layer of 

 cells. The cells of the lower third of the blastula are somewhat 

 larger, and contain more yolk granules than the upper cells. 

 The lower surface becomes flattened, and the larger cells are 

 then pushed into the blastoccele ; and this process of invagin- 

 ation is continued until the blastoccele is altogether obliterated. 

 The embryo now has the form of a two-layered cup (fig. 47, K\ 

 the cavity of which is the primitive gut or archenteron, and 



* " Amphioxus, and the Ancestry of the Vertebrates," by Arthur Willey. 

 Columbia University Biological Series. Macmillan & Co., London and 

 New York. 1894. 



