8 



N. YATSU ' OBSERVATIONS AND EXPERIMENTS 



2 Twin embryos. 



It is interesting to note that from an egg which was cut in two 

 along the equatorial plane before the beginning of the first cleavage 

 developed two embryos (Figs. 4, 5 and 6). Unfortunately I did not 

 record which fragment had the polocytes, two being reared in one 

 salt-celler. But judging from the number of comb-rows, it is almost 

 certain that the embryo represented in Fig. 5 is from the micro- 

 meric half. 



The egg must have been fertilized more than one spermatozoon, one 

 of the embryos being merogonic. At any rate the points of interest 

 regarding these twin embryos lie in the fact that both of them had 

 a well developed apical sense organ, stomodacum and two entoderm 

 sacs. From this it may be concluded that the basis of the sense 

 organ is not restricted to one hemisphere. 



3- Formation of comb-rows in the end cells 

 and middle-celt. 



To determine whether the end-cells and middle-cells behave in 

 the same way in respect to the capacity of forming of comb-rows, the 

 following series of experiments were made. 



At the 8-cell stage one end-cell was separated (Fig. 7) from other 

 blastomeres and was allowed to develop. Twelve embryos were ob- 

 tained. All of them had one row of comb-plates and no stomodacum 

 (Figs. 8, 9). 



From two end-cells (Fig. 10) I had embryos with two rows of 

 comb-plates and without the stomodaeum (Fig. 11) (six cases). 



Two middle-cells were cut out at the 8-ccll stage (Fig. 14). The 

 resulting embryos (five cases) showed a great deal of variations. In 

 one of them two comb-rows were formed (Fig. 15) as we might 

 anticipate. In another embryo a very small group of comb-plates 

 was developed. In three no comb-rows were formed at all. One of 



