PLATE XL. 



Fig. 1. P.[araHchthys] oblongus. [be = blastocoele; ep = epidermis; pb = periblast]. 61 sec- 



(92) tioiis. This figure is the twenty-fifth [of sixty-five sections] and is hke the middle ones. The 

 periblast is thin but extends farther under the cap than in Fig. 2. Near the center the layer 

 fades out and is not recognizable, but four nuclei are very distinct, showing that the layer exists 

 though thin and perhaps not fully differentiated. The nuclei extend under the entire cap 

 although the ring has scarcelj' begim. 



This shows that the periblast has already begun to expand as an independent layer and has 

 therefore probably ceased to contribute directly to the cap. 

 F"ig. 2. P. oblongus. [be = blastocoele; ep = epidermis; pb = periblast). Three hours after 



(91) 32-cell stage. Four of sixty nearly longitudinal sections. Left end = embryonic region. 



The second section does not reach cap; third touches cap. 



The fourth section shows some of cap, and periblast thinner at middle. 



The eleventh section shows periblast still thinner in middle. Nuclei arc more numerous at 

 left end all through these sections. I think the pcriblastic nuclei are more numerous under 

 region of embryo than elsewhere. 



On the seventeenth section the periblast is scarcely visible at center beneath cap. On the 

 sixteenth two nuclei were found near the middle of the subgenninal plate, although the plate 

 is scarcely traceable. 



One nucleus was seen near middle of the nineteenth section and one in twenty-fourth. 

 The periblast is somewhat less in bulk on the middle sections than on the thirty-eighth which 

 I have drawn. Periblast is here about same in quantity as in Fig. 2, Plate XXXIX. (Three 

 hours after 32-cell). 



