No. 2.] EMBRYOLOGY OF THE AMERICAN LOBSTER. 243 



yolk. A lumen free from yolk does not occur in the lobster, 

 though the more amoeboid cells certainly attack the yolk. Ho- 

 marns and Astacus agree in having the mesodermal nuclei in the 

 neighborhood of the blastopore, considerably larger than else- 

 where. Reichenbach claims for the anus a position anterior to 

 the blastopore. In Homarus it does not occupy, always, the same 

 position. It may arise from the anterior, middle, or posterior 

 portion of the scar that is left after the closure of the blasto- 

 pore. In both forms, it at first leads into a mass of mesoderm, 

 and only later reaches the entoderm sac. I have been unable 

 to find, in Homarus, preparations that throw any direct light on 

 the so-called "secondary mesoderm." 



Stage K. 



A surface view of fresh eggs that have been treated with ten 

 per cent nitric acid shows that the cloud-like tracts of J have 

 now assumed the form of definitely and abruptly elevated ridges 

 and hills. These are considerably smaller in superficial area than 

 the earlier tracts, because of the tucking under of the ectoderm 

 to elevate the appendages. The antennae, in relation to the 

 stomoclaeum, have moved slightly anteriorly. Though the pro- 

 cephalic lobes have become considerably contracted, their former 

 bounds are still marked by the action of the acid. A broad cloud 

 extends backward from the thoracico-abdominal plate. 



The stained egg is shown on PI. XVIII, Fig. 3. The outlines 

 of the appendages have become more definite and the nuclei 

 more abundant. The contraction of the embryonic area is most 

 striking, and the procephalic lobes no longer show the general- 

 ized condition noted in the preceding stage. The structure of 

 the anterior portion of the embryo is with difficulty demon- 

 strated. From examination of many preparations I conclude 

 that the optic ganglia (G. 1 ) are in close contact with the optic 

 tract (£>.), while the cerebral ganglia (G. 2 ) have retained their 

 more primitive condition, and are connected with the optic 

 ganglia by a band of larger nuclei (Nc.). 



All traces of the crescentic appendages have disappeared. 



The ganglia of the antennas are still prominent hemispherical 

 swellings, but they now occupy a somewhat pre-oral position, 

 and are closely associated with the supra-cesophageal swellings. 

 The antennae, curiously enough, point anteriorly, and a break 



