No. 2.] EMBRYOLOGY OF THE AMERICAN LOBSTER. 241 



pore. The lateral pressure which brought them together seems 

 to continue for some little time, and finally results in a central 

 elevation, which continues to be a feature of this and the next 

 succeeding stage. As growth in this region continues, the 

 blastopore, occupying in relation to the fold a more or less 

 posterior position, is actually closed, though the arrangement 

 of its nuclei makes it possible, for one to follow it into the later 

 stages. In the present stage the blastopore may be seen in 

 both longitudinal and transverse sections. 



A groove in certain eggs passes from between the ganglia of 

 the jaws far forward and almost to the mouth. At times, too, 

 the nuclei behind the blastopore arrange themselves in parallel 

 median lines. In PI. XVII, Fig. 1, the entoderm is seen below 

 the surface at Eut. Although the elaborate system of curves 

 described by Reichenbach is at times suggested, continuous 

 lines of nuclei are generally very short. 



The present stage in Homarus is more directly comparable 

 with E and F of Astacus, but the internal structure combines 

 many features of D and E. The surface development of Astacus 

 is, when compared with the interior growth, retarded. Whereas 

 the nauplius appendages of Homarus are now present, though of 

 varying size, in Astacus they are not as yet differentiated. The 

 procephalic region is quite similar in both forms, though the 

 optic ganglia in Astacus are farther removed from the cerebral 

 or supra-cesophageal. The anus has already made its appearance 

 in Astacus. 



A longitudinal section shows the even contour of the younger 

 embryos to be more or less broken (though normally less than 

 figured PI. XVII, Fig. 5, where the reagents have contracted 

 the embryonic surface). The blastopore is no longer a depres- 

 sion, and in longitudinal sections is difficult to find, though indi- 

 cated in Fig. 5 at G.M. Passing anteriorly, a considerable 

 thickening of the ectoderm occurs at G. 5 , marking the position 

 of the associated ganglia of the jaws. Though the cells are 

 deep, but a single line of nuclei is present. Below this point 

 chromatin grains are especially abundant. A few mesodermal 

 cells are also shown (Mes.). Behind the mouth (St.) the ecto- 

 derm is relatively thin, though at the stomodasum it thickens 

 again and is covered below by a number of mesodermal cells. 

 Chromatin granules are very abundant under the procephalic 



