GERM DEVELOPMENT IX THE LANCELET. 447 



appears, the two primary germ-layers of the Amphioxus 

 larva split up into the four secondary germ-layers (Fig. 157, 

 transverse section). Round the inner vegetative layer of 

 the intestinal tube there arises, in consequence of a fission 

 of the cells of the latter, a second external cell-stratum, the 



FIG. 157. Transverse section through 

 u larval Amphioxns (after Kowalevsky) : 

 /is, skin-sensory layer ; hm, skin-fibrous 

 layer; c, ccelom-fissure (rudimentary 

 body-cavity) ; df, intestinal-fibrous layer; 

 dd, intestinal glandular layer; a, primi- 

 tive intestine (primitive intestinal cavity). 

 Above, the dorsal furrow is seen between 

 the two dorsal swellings. 



intestinal-fibrous layer (df) ; from this originate the 

 muscles and the fibrous membranes of the intestinal tube, 

 and the blood-vessels. The original inner cell-stratum 

 must now be called the intestinal-glandular layer (dd). 

 Analogousry, the outer animal germ-layer falls, in con- 

 sequence of a fission in its cells, into two strata, an outer 

 skin-sensory layer (hs~) and an inner skin-fibrous layer (km). 

 The former gives rise to the outer skin (epidermis) and the 

 medullary tube ; the latter to the leather-skin (coHum) 

 and the trunk-muscles. A space forms between the skin- 

 fibrous layer and the intestinal-fibrous layer, in which a 

 colourless liquid collects, thus forming the body-cavity 

 (cceloma, c). It is a fact of great moment for the germ- 

 layer theoiy that, here in the Amphioxus, the origin of the 

 skin-fibrous layer from the animal, and that of the intestinal- 

 fibrous layer from the vegetative germ-layer is plainly 

 demonstrable. 



As soon as the four secondary germ-layers have formed 



