EXPERIMENTAL PRODUCTION OF TWINS 21 



and those at the thinner pole are free from yolk. Obvi- 

 ously then we have a well-defined animal and vegetal 

 pole, and gastrulation or archenteron formation is an in- 

 growth or invagination of the vegetal pole (Fig. 10). In 

 the case of parthenogenetic blastulae, however, numerous 

 deviations from the normal conditions are observed: 



a) Some blastulae remain nearly solid, showing 

 scarcely any cleavage cavity. Such blastulae never 

 gastrulate at all. 



b) Other blastulae (Fig. 11) are without any polarity. 

 The cleavage cavity is large but the yolk material is 

 evenly distributed among all of the cells. Such blastulae 

 usually undergo multiple gastrulation, the surface in- 

 vaginating intricately as in Figure 12. 



c) Other blastulae, instead of having one thickened, 

 yolk-laden region of the blastoderm, have two or more 

 such regions (Fig. 13). Such bipolar and tripolar 

 blastulae invaginate at two or three places to make the 

 types of larvae in which there are two or three archen- 

 tera. A typical bipolar gastrula is shown in Figure 14. 

 If the original polarity is retained to some extent and 

 only a relatively small, thickened, yolk-laden area 

 appears at the opposite pole, we get gastrulae and bipen- 

 nariae of a bipolar sort with a supernumerary archenteron 

 at the original apical end. Sometimes the thickening 

 at the apical end fails to invaginate and looks somewhat 

 like an ''apical plate." It has, in fact, been so inter- 

 preted (see Heath, 1906). 



d) Sometimes the thickened basal area becomes much 

 broader than normal and has a thinned-out region in the 

 middle, as though a sort of fission of the vegetal pole had 

 occurred. Such blastulae produce true identical twin 



