64 THE AMPHIOXUS. 



mode we have mentioned of the closing of the gastrula 

 is the simplest mechanical process by which the one 

 form can be changed into the other. In this way, 

 without pre-supposing any important cell displace- 

 ments, is to be explained the change of the broad, 

 wide-open, flat, cap-formed gastrula, into the con- 

 siderably diminished form of Figs. 33, 34. This process 

 can be easily represented by a flexible model. 



We arrive also at the same conclusion on a careful 

 comparison of the stages. Comparing Fig. 26 with the 

 earlier stage of Fig. 24, we see that the originally short 

 dorsal part has become considerably lengthened. At 

 the same time the flattening of the dorsal surface 

 appears more prominent. In the further stages also, 

 Figs. 29 and 31, we see that the dorsal side becomes 

 continually longer and longer, while the ventral 

 side of the curvature shows only an alteration of its 

 curve occasioned by diminution of the gastrula-mouth. ' 

 To this diminution of the gastrula-mouth it is also due 

 that the angle at which the ventral and dorsal walls 

 meet one another towards the anterior end becomes 

 continually smaller and smaller, until at last the dorsal 

 surface gains a direction parallel to the longitudinal 

 axis (Fig. 33). 



Other circumstances may also be observed, and the 



