18 K. MITSUKURI AND C. ISHIKAWA. 



with the time when the eggs are deposited, aud endiug with 

 their hatching out. The present paper gives the results of our 

 study on the formation of the germinal layers. Papers on 

 other points and later stages of development will follow from 

 time to time since the investigation is being continued, as the 

 pressure of other duties permit us. 



We wish to return our warmest thanks to Mr. Hattori for 

 cheerfully acceding to our numerous demands on his good 

 nature, and for furthering greatly our work with his intelli- 

 gent assistance. Thanks are also due to the authorities of the 

 University of Tokyo for the payment of necessary expenses 

 attending the investigation, and for the use of instruments, 

 reagents, &c. Finally, we wish to express our deep obligations 

 to Dr. Isao Jijima for valuable suggestions in regard to the 

 methods of investigation. 



We made many interesting observations on the breeding 

 habits of Trionyx, but we reserve these for some other occasion, 

 as foreign to the purpose of this paper. We simply mention 

 that the Trionyx eggs are nearly spherical in shape, and have 

 a hard brittle shell like that of the fowl, and not leathery, as 

 in some Chelonia. Their size is very variable, the smallest we 

 measured being 10 mm., the largest 23 mm., the most usual 

 size about 21 — 22 mm. in diameter. This difference in size 

 seems to be due mostly to the size of the parent. With this 

 we pass on at once to the consideration of the subject proper 

 of the present communication. 



The earliest stage of which we will give a detailed description 

 is taken from an egg opened directly after its deposition. Our 

 attempts to obtain still earlier stages by opening pregnant 

 females have proved but partially successful. In almost every 

 case, with only some doubtful exceptions, the eggs we found 

 in the oviduct were unfortunately fully as much advanced as 

 those just laid. 



On opening an egg directly after its deposition the blasto- 

 derm is always found at the pole turned above. The embryonic 

 shield, with the pellucid area around it, stands out conspicuously 

 as a siuall, nearly circular spot, on the yellow surface of the 



