430 MEMOIRS OF THE NATIONAL ACADEMY CF SCIENCES. 



Ishikawa (27) finds in Atyephyra after the close of the invagination stage, certain proto- 

 plasmic elements under the ectoblast, which he thinks may correspond with the" white yolk ele- 

 ments'' just referred to, and he also identifies "secondary mesoderm cells," but does not trace 

 their origin or function. They are " small granules," easily stained by logwood solution, and some 

 are of considerable size and have a clear cell outline. "These are mostly aggregated in the 

 cephalic region between the involutions of the ectoderm cells, but are also found in all places." In 

 time of appearance and ill their position, he says they seem to correspond to the " secondary meso- 

 derm cells" of Astacus. This short notice with his figures leaves little doubt that these bodies are 

 similar to those just described in Alpheus and Homarus. Fig. G2 of his paper represents a longitu- 

 dinal median section of the egg-nauplius, and may be compared with the same stage of Alpheus 

 (Figs. 104, 105), with respect to the general character and appearance of the degenerating cells. 



I have noticed similar nuclear fragments in the egg-nauplius of a crab (Fig. 113, PI. XL), and 

 Lebedinski (34) has described "secondary mesoderm" in the embryo of the Mediterranean, sea 

 crab, Eriphia njiini/'roiis. According to this observer they are found in all stages from the "gastrula" 

 on, to the egg-uauplins ; they are derived from ectoderm, and probably give rise to blood cells. 

 In the stage with one pair of maxillipeds these elements are in active proliferation : 



M;iu iindet, die Ze.lleu desselben bieten verschiedeue Mouieuto and Zustande des Zerl'allens dar; dieses Zerfalleu 

 iler /.ellon steht in geuauou Zusamiuonhange mit der Eutstehnng dor Blutkiirptr. 



lie further says : 



Ueber dio Bildiing des Blnteskanu ich nichts bestimmtes luittheileu. Ini Stadium drsi'iNtcii 1'aars kiefori'iisscheu 

 siud die ersten Blutkoiperchen vorhandou, welclie zuiu ersteu Mai iui Beroiche des Herzens vorkommen, wosiuh auch 

 am friihesten das sekunduro Mesoderm riickzubilden beginut. 



From these quotations it appears that the " secondary mesoderm " shows .signs of degetiera- 

 tiou, and its conversion into blood cells is au unverified inference. It seems more probable that 

 the structures in question correspond with similar bodies already noticed in Alphens, riomarus, 

 and other Decapods, and that in all cases they have to do primarily with the dissolution and not 

 with the construction of cells. 



\Vheeler (67) in his careful paper on the development of the Cockroach and Potato beetle 

 (Blatta germanica and Doryphora decemlineata) describes au interesting case of the decomposition 

 of nuclei, which bears a close analogy to what takes place in Alpheus and probably also in 

 Astacus. In Doryphora two masses of endoderm are found, one under the stoinodaeum the other 

 under the caudal plate. At both these places numerous cells which originate in the endoderm 

 pass into the adjacent yolk and disappear. The process of dissolution is described as follows : 



The karyochyletua becomes vacuolated, probably with substances absorbed from without, to judge of the larger 

 size of some of these nuclei, while the chromatin ceases to present the threadlike coil and becomes compacted into 

 irregular masses between the vacuoles. Finally the vacnoles fuse and the masses of chromatin, formally numerous, 

 agglomerate to form one or two largo irregular masses which usually apply themselves to the wall of the clearly vesic- 

 ularnucleus * * In the last stages seen the masses of cbromatiu lie between the yolk bodies, all other portions 

 of the nucleus having disappeared. They still take the deep red stain, but finally become comminuted and disappear 

 in the iutervitelliue protoplasm. 



The vesiculated elements recall similar bodies which appear in Reichenbach's plates. Thus 

 the element t, Fig. 88 of Wheeler's paper, where the chromatin is applied to the walls of the 

 nucleus, strikingly resembles nucleus i, Fig. 20 (see this paper), where the chromatin is similarly 

 disposed around the wall of a vacuole. 



Bruce (10) figures certain yolk cells undergoing what he considered to be endogenous cell 

 division in an advanced embryo of a spider, and compared it with the endogenous cell division 

 which Reichenbach describes as taking place in the endoderm cells of Astacus. 



The disintegration which has been attributed to the leucocytes of the mammalian blood 

 affords au interesting comparison with the phenomena which have been described for the Arthropod 

 embryo. Howell's careful observations (25) support the view that the multiuucleated leucocytes 

 are disintegrating cells. " The leucoblasts enter the lymph stream, and Eventually reach the 

 blood as unicellular leucocytes." Here they undergo changes, acquire amojboid movements, while 

 the nuclei elongate, become constricted, and finally fragmented. " The mnltinuclear stage * * * 

 is probably followed by a complete dissolution of the cell." Howell adopts the highly reasonable 



