47o THE LANCELETS— CEPHALOCHORDA 



in the walls of its nerve-tube photo-receptors, which may well be 

 the forerunners of the rod and cone-cells of the vertebrate retina. 

 It is thus an animal of fundamental importance for the under- 

 standing of the vertebrate sense-organs " (Parker). 



Development and Life-history 



Development. — The sexes are hardly distinguishable. There 

 are twenty-six pairs of simple sacs — ovaries or testes — projecting 

 into the atrial cavity. The eggs of Amphioxus are transparent 

 spheres about o|o m - m diameter. They pass out of the atriopore 

 and are fertilised in the water. The segmentation is complete 

 and almost equal. As if cut by an invisible knife, the egg 

 divides vertically into two cells. Another vertical cut, at right 

 angles to the first, leads to the four-cell stage. The next cut 

 is in a horizontal plane equatorially, at right angles to the two 

 others ; and thus eight cells are formed. But the four cells in 

 the lower part of the egg are larger than the upper four. As 

 division goes on, a hollow ball of cells is formed, a blastula or 

 blastosphere, and by a process of unequal growth one hemisphere 

 is indimpled or invaginated into the other, as one might in- 

 dimple a punctured india-rubber ball. Thus there is formed a 

 two-layered embryonic stage, known as the gastrula — of very 

 wide occurrence among animals. The outer layer is called 

 ectoderm or epiblast ; the inner layer endoderm or hypoblast, 

 and the cavity is called the primitive gut or archenteron. 



The gastrula elongates and its aperture (the blastopore) be- 

 comes narrower. A flattening occurs along the dorsal median 

 line, the ectodermic part of which is the foundation of the nerve- 

 cord, while the endodermic part is the beginning of the skeletal 

 notochord. The ectodermic plate becomes a groove (the neural 

 or medullary groove) which is over-arched in a quite unusual 

 way by a growth of the lateral ectoderm. Somewhat later the 

 neural groove closes in of itself to form a neural canal, which 

 opens posteriorly (via the blastopore) into the gastrula cavity, 

 and anteriorly to the exterior by a minute hole (the neuropore). 

 The cavity of the gastrula becomes the gut : along its roof 

 the foundation of the notochord is seen at a very early stage ; 

 and pouches grow out which form the middle embryonic layer 

 or mesoderm and the primary cavities of the body. 



As is common when there is very little yolk, the develop- 



