MEMOIRS OF THE NATIONAL ACADEMY OF SCIENCES. 429 



Reicheubach thus summarizes his observations on the "sekundiire Mesodermzelleu :" 



Die fraglichen Elemeuto sind als Zelleu zu deuteu, dereu Kerno uicht iuiuier die BesohaffeDheit gowohnlicher 

 Zellkerue liabou, diesclbo aber friiher odor spiitor orlaugen (Fig. 'M, in, in-, Plato xxvn). Sio ucliiuen ibrou Ursprung 

 iuuorhalb dorjonigeu Eutoderuizellen, welche dio ventralo Waud des Urdarmaackchens zusamiiiensotzeii durch eino 

 niiher zu erforgchendo Art eudogener Zellbidung, bei wolcher die in dor Mehrzahl in don Eleineuteu des Entudorins 

 vorhandeuon Kt-rno cine wichtige Eolle zu spielen scheiuen. In don doin Stadium D vorangehenden Entwicklung- 

 speriodou bat jodo Eutodermzelle uieist nur einen Kern ; diea trifft auch uoch zum Thcil fiir Stadium D zu. Bald ver- 

 meliren sich :iber die Entoderiukorne ganz erhoblich mid oudlicli beginuen die sekundiirou Mesodermzellen aufzutreteu. 

 Weiin eine grossore Zahl der sekuudareii Mesodermzelleii in den Entodermelementeu liogeu, so scbeint das Kernma- 

 terial verbraucbt zn sein. Es wandern nun aller Wahrscheinlichkeit nacb dieso Zellun, deren Kerue anscheineud 

 noch in dor Metamorphose sich befinden, aus deui Eutoderm aus uud begebou sich uuter die Embryoualanlage. Die 

 tetreft'eudou Contouren. des Entoderms lassen oft uoch Spuren dieaer Wanderung erkeuneu. Ob sie wirklich aktiv 

 aiiswaudern odor anch ausgestosson werdeu, ist nicht festzustelleu gewosen. Sie begobeu sich nun unter die ubrigcn 

 Mosodermzelleu uud Bind bald uicht mehr von ihuen zu uuterscheidou. Aus dieseiu Grinul fiihrto ioh liir sie deu 

 Nameu "seknndare Mesodertnzelleu" eiu, wiihrend die altereu Urmesodermzelleu als primilre bezeichuet werdeu. 

 Da die letztoron dio Tendenz zeigen, zu kompakteren Massen zu verwachsen, so darf man wohl vermuteu, dasa die 

 sekuudiiren Mesodermzellen die Blutzellen liefern werden (54, p. 36). 



It is interesting to notice that in Alpheus, Astacus, and Hotnarus degenerating cells appear 

 in greatest force at about the egg-nauplius stage, and from that time ou their numbers begin to 

 waue. In Astacus, Eeicheubach first noticed the " sekuudiire Mesoderuizelleu " in stage D (that 

 is, when the optic disks, the thoracic-abdominal plate, aud the mandibles are outlined), which nearly 

 corresponds to Stage IV of Alpheus. In stage U the bodies in question arc most abundant under 

 the optic disks (Kopflappeu) and in the region of the upper lip, but become more generally dis- 

 tributed in the egg-nauplius. 



According to Reichenbach, " gastrulation" takes place after the optic disks are formed, but 

 unfortunately his paper is incomplete at a very important period, namely, from the late yolk-pyra- 

 mid stage of segmentation, when the protoplasm is at the surface, to the time when an embryonic 

 disk or plate (Eutodertuhiigel or Entodermscheibe) has been formed. It is impossible, therefore, 

 to follow the history of the so-called " white yolk elements." He says of the latter : 



Sie bestehen aus protoplasmatischer, feinkornigor Substauz uud euthalten vacuolenartige Einschliisse, die ilmeu 

 ein schanuiiges Ausseheu gebeu ; ich babe sie als woisse Dotterelemepte bezeicunet. Sie liegeu entweder dicht unter 

 dem Blastoderm oder im Centrum des kugligeu Eies uud verscbwiudeu sebr bald (Op. oit., p. 7). 



According to my view these bodies ^correspond to the vesiculated elements (m, Fig. 20), and 

 both represent cells in process of dissolution. If Figs. 18 aud 20 are compared (the latter being 

 a copy of Reichenbach's Fig. 67) we will find a striking correspondence between these peculiar cell 

 products iu both Astacus and Alpheus, a correspondence which is even more marked when the 

 comparison is made with the lobster. 



Reichenbach emphasizes the statement that naked balls of nuclear material never occur free 

 iu the yolk outside the endodermal sac. In this yolk they always have a "cell body" as K, Fig. 

 20. It is an easy matter, however, to distinguish naked masses of chromatin, minute spherules, 

 or smaller granules iu the yolk of both the lobster and Alpheus. 



My studies of the lobster are not yet completed, but from the observations which have already 

 been made I draw the following conclusions: In Alpheus, the lobster, and the crayfish similar 

 bodies make their appearance at nearly similar times and play a similar role. They are derived 

 from all three layers of the germ, and in Alpheus minor degenerative products make their appear- 

 ance in the segmentation stages. They tend to break up aud ingest the yolk and to produce iu it 

 a chemical change, possibly in order that it may be more easily assimilated by the other embry- 

 onic cells. Having performed this task they degenerate ; they are converted into a substance 

 resembling yolk aud function as nutritiou. That any play a formative role, giving rise to blood 

 cells for instance, as Reichenbach supposes, there is no direct evidence. The vitellophagous func- 

 tion seems to be iu abeyance in Alpheus, but iu all cases the yolk is comminuted and chemically 

 changed in the neighborhood of these bodies. Nusbauin (45), following Moriu, believes that the 

 "white yolk elements." arise from the segmentation nucleus and migrate to the surface of the egg; 

 that they give rise to the "secondary mesoderm." which are taken up along with the yolk by the 

 anueboid, endoderinal cells. This is reversing the account, and, iu so tar as the origin of the "sec- 

 ondary mesoderin" is concerned, it is not supported, so far as I am aware, by a single observation. 



