238 RUMPUS. [Vol. V. 



and in every way agrees with the longitudinal : the mesoderm 

 cells extend in a sheet to the right and to the left of the median 

 line. Reference to Fig. 5, which represents a cross-section, 

 will show the thickenings of ectoderm where the elevations of 

 the jaws are soon to appear. Karyokinetic figures in the ento- 

 derm prove that cell multiplication is there rapidly going on, 

 and at K.f., Fig. 6, give us some idea of the limits of an ento- 

 derm cell. The section (Fig. 4) passes at right angles through 

 the procephalic mass. It shows the forward extension of the 

 chromatin nebulae and of the mesoderm, the nuclei of the lat- 

 ter appearing in a fine-grained mass recalling the " serum " of 

 Reichenbach ('86). The curve of the section is seen to be 

 abruptly broken at * — *, which are the early depressions lateral 

 to the procephalic lobes. The position of the equatorial plate 

 at K.f. proves that the ectoderm cells are still multiplying hori- 

 zontally. In not a single instance have I found these bands 

 placed otherwise in ectoderm cells. 



So far as the surface is concerned the present stage of 

 Homarus combines the stages B, C, and D of Reichenbach. 

 At no stage, however, does the lobster show the regular nuclear 

 curves of Astacus, nor is it possible to demonstrate any differ- 

 ence between the several cells of the procephalic lobes compar- 

 able to the primitive " Krystallkegelzellen " of Reichenbach. 

 The irregularities in methods of closure of the blastopore of the 

 lobster are duplicated in the cray-fish. In stage B of Reichen- 

 bach the so-called secondary mesodermal elements are introduced. 

 Future comparison may prove these to be the same as the 

 plasma vacuoles and their chromatin grains. The invagination 

 that results in the formation of entoderm in Astacus is some- 

 what different from the proliferating mass described for Homa- 

 rus, though certain sections which I have made show that not 

 infrequently the entoderm of Homarus may contain an indistinct 

 lumen, though I have found no continuation of this entodermal 

 cavity to the surface of the egg. (This structure has already 

 been noted by Herrick, '90, in his preliminary paper on the 

 development of the present type.) The entoderm of the cray- 

 fish is inclined anteriorly, while in the lobster it tends most 

 generally towards the posterior, and in certain cases is actually 

 elevated so that it reaches the ectoderm behind the blastopore. 

 Fig. 13 represents the entoderm in its most normal condition. 



