No. 2.] EMBRYOLOGY OF THE AMERICAN LOBSTER. 23 1 



there difficulty in distinguishing the two portions of the seg- 

 menting egg. The furrows between the blastomeres now extend 

 deep down into the substance of the egg, though there is always 

 a central mass of yolk which remains undivided. Careful obser- 

 vation will show that some of the blastomeres are in a process of 

 still further subdivision though the nuclei have not yet reached 

 the surface so that they can be seen in the living egg : they 

 may, however, be clearly brought out by the action of reagents. 



Polar globules are to be seen at R-k, Fig. D. A cross- 

 section (PL XIX, Fig. 3) shows that all the nuclei have reached 

 the outer portion of the egg, and a careful examination of com- 

 plete series through several eggs failed to show any different 

 position for these nuclei, or rather cells, since each nucleus is 

 surrounded by an amoeboid mass of protoplasm. The centre 

 and periphery seem to take the stain more readily than inter- 

 mediate portions of the egg. 



Though the morula stage just described is adopted by the 

 majority of eggs, some pass over it in a most irregular man- 

 ner. Fig. F, (PI. XIV) represents a case where the cells of the 

 animal pole have divided very rapidly and are enclosing the 

 vegetative cells as by a process of epibole. The nuclei have in 

 both cases reached the surface, and are surrounded in living 

 eggs by whitish clouds. 



The Gastrula. 



On the third or fourth day after oviposition, the blastoderm 

 is found to be invaginated as shown in PI. XIV, Fig. G. The 

 blastopore is generally elongated, though I have found it quite 

 circular and at times triangular. It also presents considerable 

 variation in size. A view of the opposite pole shows that the 

 blastoderm cells are there larger, and through their transparent 

 bodies the yolk balls may be seen within. 



The next following day shows the blastoderm cells smaller 

 and more numerous, and the addition of weak nitric acid to the 

 sea-water shows the blastopore to be the probable centre of 

 activity of the developing egg. 



Stage H. 



Seen from above, the blastopore at this stage normally appears 

 as an oval depression, of which the major axis extends from left 



