428 MEMOIRS OF THE NATIONAL ACADEMY OF SCIENCES. 



characteristic is certainly not so apparent iti Alplieus, yet I believe that in this form it is present 

 in some stages, though in a less marked degree. 



Astacus (?) I have studied several critical stages in the development of the crayfish, critical at 

 least so far as the appearance of degenerative products is concerned, with an eye to comparison 

 with Alpheus and Hoinarus.* The youngest set of embryos corresponds nearly to Reicheubach's 

 stage E, but differs from it iu some details. Rudiments of five pairs of appendages are present, 

 the two maxillae being seen between the maudibles and the thoracic-abdominal process. None 

 of the appendages, however, are folded. The mouth is seen on a line between the first and second 

 pair of antenna?. 



The bodies which Reichenbach calls "secondary mesoderm" occur iu abundance in or near the 

 wall of the endoderm sac next the embryo. They also abound iu the yolk under the ectoderm, 

 and are most numerous in the area extending from the optic invaginations to the mouth or slightly 

 behind it. In this respect they recall the distribution of similar bodies in Alpheus and Ilomarus. 



I wish to call attention to the fact that at this stage none of my sections show a cavity in the 

 endoderm sac, as is represented by Reichenbach (compsfre Taf. vm, 54), and the endodermal yolk 

 segments or pyramids do not always possess completed walls. To what extent this appearance is 

 normal, and to what extent due to the action of reagents, I can not at present say. These eggs were 

 treated with hot water and corrosive sublimate. The eudodermal nucleus is surrounded by a thin 

 layer of protoplasm, which works its way amid the yolk so as to practically surround a pyramidal 

 mass. This strongly recalls the serpentine manner iu which the endoderm cells creep through the 

 yolk iu Ilomarus. Whether these cells in Astacus are simply migrating iu a column or sheet, 

 spreading gradually towards the periphery of the egg, as in the lobster, cannot be decided from the 

 material at my command, but it is a point of considerable interest iu its morphological bearings. 

 The eudodermal cells probably multiply indirectly, but I saw no nuclear figures in my sections, 

 and they appear also to divide directly, independently of the yolk pyramids as Reicheubach 

 has described, giving rise to the chromatiu balls and granulated elements (compare Fig. 20) but, 

 as pointed out above, this appearance may be very deceitful. .This process is most marked in the 

 endodermic area noticed above, underlying the anterior half of the embryo. Here we see great 

 numbers of the bodies of varying size, both within and without the domain of the endoderm cells. 

 They closely resemble the vitellophagous elements which I have described for the lobster, and 

 possibly they attack the yolk in a similar way. Where they are thickest the yolk is comminuted 

 and shows traces of profound chemical change. In the midst of the altered yolk one can discover 

 very faint outlines of vesicular bodies which exhibit but slight reaction to the stain. These I 

 regard as degenerated cells. 



The next stage of Astacus which I have studied corresponds nearly to Reichenbach's stage G. 

 Eight pairs of appendages are present, and there are rudiments of a ninth pair. The first and 

 second maxillae appear as distinct buds, while the third pair of maxillipeds is represented by a 

 proliferating cell area only. The extremity of the abdomen is not bifid. 



In the central part of the endodermal sac there is a coagulable fluid which comes iu close rela- 

 tion with the posterior end of the embryo. The endodermal cells laterally have quite or nearly 

 reached the ectoderm, while dorsally they fall a little short of the surface. The yolk within the 

 (tontines of the endoderm has an irregular, pyramidal, or radial cleavage. Centrally the yolk 

 blends with the serum-like fluid, in which occasional granules or balls of chromatiu may be found. 

 Small spherical elements (like those represented iu Fig. 18, , fc, c, or A-, Ar, 1 Fig. 20), containing a 

 single chromatiu ball or several balls, occur not only in the yolk underneath the ectoderm and in 

 the vicinity of the endodermal nuclei, but also in the central yolk of the endoderm sac, at various 

 levels below the endodermal nuclei. This is a point of some iuterest in connection with the fate 

 of these bodies. They wander not only peripherally but centrally. Rarely we meet one which is 

 three or four times the average size, having a small chromatin spherule iu its center. In later 

 stages they are present in far less numbers. 



* For tlio opportunity of studying tlu< crayfish ilovelopiuent at this time I am indebted to the kindness of my 

 friend, Dr. William Patten, who sent rne a number of important Nta.urs collected at Milwaukee. 



