No. 2.] EMBRYOLOGY OF THE AMERICAN LOBSTER. 237 



shallow divergent furrows extend anteriorly and laterally, en- 

 closing the more highly nucleated procephalic portion. It is in 

 the neighborhood of these furrows that the protoplasm of the 

 fresh egg, on treatment with nitric acid, yields the whitish 

 indefinite ends of the U-shaped cloud. 



A longitudinal section through the median line shows the 

 blastopore to be deeper and more constricted than in the pre- 

 vious stage. The ectodermal cells have again increased in 

 number, especially in the region of the head folds (Fig. 13, 

 Pel.), where even in the middle line the juxtaposited columnar 

 cells attract the eye. Midway between the head folds and the 

 blastopore, as has been observed for earlier stages, the ecto- 

 dermal nuclei are far apart, though immediately in front of 

 the invagination, — in the thoracico-abdominal portion, — they 

 are again aggregated. Chromatin grains are still present as 

 nebulae (C.n.), most abundant under the portions of the ecto- 

 derm that have been noted as areas of special growth. A few 

 grains are noticed near the entoderm. 



The entoderm cells {Ent.) have increased in number with 

 the increase of the ectoderm cells. They extend deep into the 

 yolk, as Herrick ('90) has already stated, as a " flask-shaped " 

 mass. At places pseudopodia-like prolongations are seen extend- 

 ing out between the yolk bodies. 



That which specially characterizes the present stage, how- 

 ever, is the appearance of mesodermal elements. In the longi- 

 tudinal section at . Mcs. is to be seen a small group of lighter 

 colored nuclei which tend anteriorly. 



The mesoderm nuclei are at this stage lighter in color and 

 generally more oval in outline than the darker more angular 

 entoderm elements. They closely resemble the ectoderm nuclei, 

 and their origin is clearly seen to be from the point where the 

 latter layer fuses with the entoderm, though the sections do 

 not show, what Ishikawa and Reichenbach have figured : cells 

 mifrratinof from the ectoderm into the mesoderm. A few meso- 

 derm cells are to be seen at Mcs. just in front of the blastopore 

 and beneath the ectoderm. In the lower, deeper portions of the 

 entodermic mass no such cells, nor even nuclei, are to be found. 

 The greater number of chromatin grains will be found to 

 accompany the growing mesoderm. 



A cross-section through the blastopore is shown at Fig. 6, 



