MEMOIKS OF THE NATIONAL ACADEMY OF SCIENCES. 429 



Reicbeubach thus suinmames his observations on the "sekuudsire Mesodermzellen :" 



Die fragliubou Elemeuto siuU aU ZoUeu zu duuteu, dureu Kerne uicht imiiiur die Bescliaffeuheit gewuhulicber 

 Zi-Ilkeriio li;»l)nii, diesulbu aber friibi-r odor sp.'iter orhuigeii ^Fig. tiO, in, in', Plate xxvil). .Sie uehiuen ibron IlrMiirung 

 iuiierbalb lUijiMiigeii Eutoderui/.illen, wclcbo dio ventralo Waud des Urd:iriM»;ickcbt!!i» ziiBaniniensctzon dtircb oiiio 

 nilber zu urt'oiuclicndo Art eudogeiier Zcllbiiluug, bei welcbtT dio in der Melirzulil in den Elenienten des Eiitodeniis 

 vorbaudciicii Ivirno eine wicbtigo Kollo zn spielen scbeiuen. In den deui Stadium I) vorangebeudon Eiitwicliluug- 

 sporiodoii liat Jode Eiitodermzelle ujoist uur eineii Kern ; dies tritU aucb uocb zuiu Tbi^l fiir Stadium D zu. Bald ver- 

 uiebreu sicb abor die Eutodermkerue ganz orbeblich und eudlicb begiiineu dio sokuudiiiou MosodermzoUen aufzutreten. 

 Wcnu oiuo griissero Zabl dor sekuudiireu Mcsodorujzelleu in deu Entodermelomenteu liegeu, so scboiut das Kerunia- 

 torial verbrauobt zu sein. Es wauderu uuu aller Wabrscbeiulicbkeit uacb dieso Zollcn, deren Korno auscbeiovud 

 uock in dor Metamorpbose sicb botiuden, aus dom Entoderm aus uud bcgeben sicb unter die Embryouabmlage. Die 

 V.etroft'oiidou Contouren des Eutodornia lasson oft nocb Spurou dioser Wanderung orkennen. Ob sie wirklicli aktiv 

 auHwainlern odor aucb ausgestosseu nerdeu, ist nicbt festzustelleu gewoson. Sie begebeu sicb nun untcr dio librigeu 

 Mosoderuizollen und siud bald uicbt mebr vou ibuen zu untorscbeiden. Aus diesem Gruud fiibrto icb liir sie deu 

 Nanien "sekundiiro Mosodormzellen " eiu, wiibrend die altereu Urmosoderuizellen als priuiiire bezoicbnet werden. 

 Da die letztoreu dio Tendenz zeigon, zu kompaktereu Massen zu verwachseu, so darf man wobl vermuteu, dass die 

 sekuudiireu Mesodermzellen die Blutzelleu liefern werden (54, p. 36). 



It is interestiug to uotice that in Alpbeus, Astacus, and Homarus degenerating cells appear 

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

 wane. In Astacus, Eeicheubach first noticed the " sekuudiire Mesodermzellen" in stage D (that 

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

 corresponds to Stage W of Alplieus. In stage D the bodies in question are most abundant under 

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

 tributed in the egg-nauplius. 



According to Keichenbach, " gastrulatiou" 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 (Entodermhiigel or Entodermscheibe) has been formed. It is impossible, therefore, 

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



Sie besteben aus protoplasmatiscber, feinkornigor Substauz uud enthalten vacuolenartige Eiuscbliisse, die ibuou 

 eiu scbanmiges Ausseben gobon ; icb babe sie als woisse Dotterolcmente bezoicbnet. Sie liegen ontwoder dicbt unter 

 dom Blasto<lorni D<ler im Centrum des kugligou Eies und verscbwinden -sobr bald (Op. cit., p. 7). 



According to my view these bodies correspond to the vesiculated elements {in, Fig. 20), and 

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

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

 products in both Astacus and Alpbeus, a correspoudence which is even more marked wheu the 

 comparison is made with the lobster. 



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

 in 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 in the yolk of both the lobster and Alplieus. 



Jly 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 api)earance at nearly similar times and play a similar role. Tiiey are derived 

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

 ance in the .segmentation stages. They tend to break up and ingest the yolk and to produce in 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 nutrition. That any play a formative role, giving rise to blood 

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

 tion seems to be in abeyance in Al[>heus, but in all cases the yolk is comminuted and chemically 

 ciianged in the neighborhood of these bodies. Nusbaum (45), following Morin, believes tliat 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, endodermal cells. This is reversing the account, and, in so far as the origin of the ".sec- 

 ondary mesoderm" is concerned, it is not supported, so far as I am aware, by a sia^tle obsi*jvation. 



