THE GASTRULA STAGE. 57 



the rest of the lip, and especially at its dorsal or anterior border, 

 the two layers pass gradually into each other. In the figures 

 this transition has, for diagrammatic purposes, been represented 

 as an abrupt one. 



The fully formed gastrula stage, seen in Figs. 19 and 20, is 

 reached, in the summer, in from seven to eight hours from the 

 time of fertilisation of the eggs. In the spring, according to 

 Hatschek's observations, the time taken to reach the same stage 

 is about fourteen hours. A comparison of Figs. 14 and 19 will 

 show that the gastrula, though of different shape, is approxi- 

 mately the same size as the egg itself. 



6. Development of the Embryo from the Completion of the Gas- 

 trula to the Time of Hatching. 



The completion of the gastrula stage is followed by a short 

 but well-marked and important period during which the rudi- 

 ments of the nervous system, of the body cavity, and of the 

 notochord are established, and at the close of which the embryo 

 works its way out of the egg membrane, swims to the surface of 

 the water by means of the flagella of the epiblast cells, and 

 becomes a free living pelagic animal. 



During this period it increases slightly in length but dimin- 

 ishes in breadth, so that at the time of hatching (Fig. 26) it is 

 about twice as long as it is wide. Its bulk remains practically 

 the same as before, for the mouth is not yet formed, and the 

 embryo consequently cannot obtain food from without, but is still 

 dependent for nourishment on the yolk granules contained in 

 the cells, more especially in the hypoblast cells. 



The nervous system is formed in the following manner. At 

 the time of completion of the gastrula the epiblast is slightly 

 flattened along the dorsal surface, as shown in Fig. 19, and still 

 better in the transverse section, Fig. 21. 



This flattened band of epiblast now becomes slightly de- 

 pressed, and at the same time becomes marked off along its 

 sides from the lateral epiblast (Fig. 22, NP). The lines of 

 demarcation are at first indicated by slight modifications in the 

 shape and arrangement of the cells, but soon become more pro- 

 nounced, the edges of the lateral plates of epiblast overlapping 

 the central depressed plate (Fig. 23), and ultimately meeting 



