SECOND PERIOD OF DEVELOPMENT. 57 



being pushed to that particular side which by being 

 flattened out becomes the dorsal side of the embryo. 

 According to Kowalevsky, from this stage on can be 

 recognised the bilateral symmetry. 



I myself was able to recognise the bilateral sym- 

 metry much earlier, that is to say, from the stage of 

 the completed in vagina tion. I also came to the con- 

 clusion that the original wide gastrula-mouth belongs 

 entirely to the later dorsal region, and that one part 

 of its edge indicates the hinder part of the body. The 

 closing of the gastrula-mouth proceeds, so far as I 

 could see, from front to back, and at last there only 

 remains the part furthest back. 



I will now state the results of my observations in 

 more detail. 



After the formation of the blastula is completed, 

 there is a pause in the increase of the cells in order to 

 allow room for another process which now directly be- 

 gins in the embryo : this is the process of gastrulation. 



On our next view of the blastula (Figs. 19, 20), which 

 forms the substratum of the alterations about to com- 

 mence, we see that at this stage, as from the beginning 

 of the development onwards, there is only a single 

 axis to be distinguished. At the lower pole we see 



