No. 2.] EMBRYOLOGY OF THE AMERICAN LOBSTER. 233 



minal enlargements, the procephalic lobes (Pel.). At other por- 

 tions of the egg the nuclei are less numerous, and on the oppo- 

 site side (Fig. 3) they stand far apart, and their surrounding 

 cells are so thin that the deeper yolk spheres become visible. 



The ectoderm cells are arranged in a continuous pavement- 

 like layer over the entire egg, their polygonal outlines appearing 

 as a faint reticulum, and their lower, deeper walls often marked 

 by irregularities of the finer or coarser yolk masses upon which 

 they rest. These cells are of course largest upon the dorsal 

 side of the egg, and smallest in superficial area where the nuclei 

 are most abundant. Where the egg has been crushed (PL XV, 

 Fig. 2), the broken edges show that each cell has immediately 

 below it a small mass of yolk material which it seems to grasp. 

 We then have a condition comparable with the "yolk pyra- 

 mids " described for related forms. The nuclei are oval in 

 outline, contain a reticulum and a central nucleolus, which lat- 

 ter often shows tendencies towards division. Karyokinetic fig- 

 ures are to be seen at different localities, most abundant where 

 most rapid growth is going on (Figs. 2 and 4, K.f.). 



The plasma vacuoles are represented by chromatin nebulae, 

 which generally underlie the triangular and U-shaped areas 

 already mentioned, though they are to be found as small clouds 

 between and also outside the limbs of the embryonal tract. 

 They are even found in the dorsal portion of the egg. In Figs. 

 3 and 4 they are represented by the minute dots, shown espe- 

 cially at C.11. 



Surface comparison shows this stage to be somewhat similar 

 to stage A of Astacus as described by Reichenbach (86). It also 

 closely resembles Kingsley's figure of Crangon. In Astacus, 

 however, the U-shaped tract has become more differenti- 

 ated, so that a procephalic portion is separated from the two 

 isolated halves of the thoracico-abdominal plate. In no case 

 could I follow the beautiful curves of Reichenbach for more 

 than a short distance, though the smaller size and the greater 

 number of nuclei in Homarus may account for this lack of 

 definite, regular arrangement. My figures (PI. XV, 3 and 4) 

 have all the nuclei, except those at the extreme periphery, 

 placed by means of a camera. 



Astacus and Homarus are externally quite similar in that the 

 invaginated cells form a considerable depression, the " Ento- 



