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



confirms our former conception of the shape of the unmodified 

 ectodermal cells, and offers strong evidence in favor of the 

 origin of the entoderm from the proliferating ectodermal cells 

 of the blastoporic depression. These latter cells (Ect.) deepen 

 as they approach the median line, and the nuclei are seen not 

 infrequently to occupy the deeper portion of the cell. The 

 karyokinetic figures {K.f.) show that the division of the nuclei 

 of the entodermal mass results in growth towards the centre 

 of the ovum, while the same figures in the ectodermal nuclei 

 (Fig. 6, K.f.) give rise to surface expansion. 



A section through the procephalic lobes (Fig. 6) shows that 

 the nuclei are less abundant in the median region, that there is 

 but a single layer of ectodermal cells, and that these have their 

 longer diameter parallel with the surface. Chromatin grains, 

 as was seen in the surface view, are most abundant where ecto- 

 dermal cells are most numerous. 



At this stage in Astacus mesodermal cells are present in con- 

 siderable number, and form tissue several layers in thickness. 

 The ectoderm is also, for some little distance anterior to the 

 blastopore, made up of two layers of cells. Structures compar- 

 able with the chromatin grains of the plasma cells are neither 

 mentioned nor figured by Reichenbach, though the so-called 

 "serum" may represent the region of their activity. In Cran- 

 gon also, at this stage, mesodermal and entodermal elements 

 are already formed, but the latter do not remain in a single 

 mass as they do in Homarus, but early migrate into the yolk. 

 Possibly, however, even while thus distributed, protoplasmic 

 filaments hold them in communication with one another. In 

 later stages of Homarus the protoplasmic continuity of the 

 entodermal cells is often beautifully shown. 



Stage I. 



This is one of the more important stages (PI. XIV, Fig. I). 

 The areas of special growth are now laid out, the blastopore 

 closes, and the germ layers are differentiated. 



The fresh egg that has been subjected to the action of dilute 

 nitric acid shows the blastopore as it disappears between the 

 converging limbs of the telson (Fig. I, G.m. and 71). Other 

 preparations were made before closure had been so far effected. 



