MEM01US OK THE NATIONAL ACADEMY OP SCIENCES. 443 



"vordere Hirnanschwellung" and tin- posterior the, "Seitenanschwellung," using tbe terms of 

 Krieger tind Dietl. These latter particulars accord with BiMchenbach's dese.ription of the crayfish. 

 1 have not, however, found that in Alpheus, behind the level of the first pair of antenna-, the lat- 

 eral parts (Seitenstrange) divided up into three sections. Beichenbach fnrtlier states that in each 

 segment a middle-strand invagination is found, while the ganglia of the fifth (first maxillary) seg- 

 ment has a prominent median string. 



In Alpheus I find an undoubted median ingrowth of surface ectoblast between the two nerve 

 cords. This in all probability corresponds to the mittelstrang, and, as already stated, it is most 

 marked at a stage just before or at the time of the first appearance of the eye pigment, when the 

 ganglia are separating from the integument. It is seen between the successive segments of the 

 ventral nerve cord, where a single cell or string of several cells appears to be wedged between the 

 two cords which are entirely separated (compare Figs. 151, 160). The nuclei of these cells are 

 elongate and perpendicular with the surface. They are derived by delaminatiou from the super- 

 ficial ectoblast, as the karyokinetic figures of dividing cells clearly show. These ingrowths are 

 most noticeable between the segments, and whether they form any part of the nervous system or 

 not, I have been unable to determine. In insects the mittelstrang appears to take no part in the 

 nervous system of the adult, but in the thoracic region it is converted into the chiteuous furcaj. 



According to Beichenbach the lower <esophageal ganglion in the crayfish is formed by the 

 fusion of the ganglia of the fourth (mandibular) to the ninth (third maxillipedal) segments. This 

 is probably true of Alpheus, as may be inferred from Fig. 196. 



With reference to the commissures, lieicheubach states his belief that the transverse commis- 

 sures originate in the unpaired fiber masses present in each ganglion, while the paired masses 

 give rise to longitudinal commissures. In Alpheus there appear in successive segments behind 

 the mouth a pair of punctsubstanz balls, one ball in each single ganglion. These extend forward 

 and backward, uniting the ganglia into chains and forming the longitudinal commissures. A little 

 later the transverse commissures are formed by bridging the cords between the points occupied by 

 the fiber-substance balls in the several ganglia. All these stages can be observed in the embryo 

 shown in Fig. 139, but all do not appear in the drawing. 



The origin of the peripheral nervous system is beset with many difficulties. It is often impossi- 

 ble to distinguish developing nerves from rudimentary connective tissue and muscle, owing to the 

 similarity of the nuclei of each. This is less true of muscles, which in some parts of the embryo, as 

 in the carapace, are clearly distinguishable. In view of this, Beicheubach found it difficult to say 

 whether the nerves were budded off from the central nervous system, from other ectoderm, or from 

 wandering mesoderm cells, lie thinks, however, that since many structures like the optic gan- 

 glion, the crystalline cone cells of the retina, etc., are early recognizable, the nervous system is 

 generally in intimate connection with the other organs. So the appendages, the nervous system, 

 and the nerves, which at first can not be distinguished from each other, are intimately connected 

 from the start; so the eye and the ear are never separated from the central nervous system. The 

 separation of the central nervous system from the skin takes place slowly, and the ectodermal 

 thickenings in the, early stages must conceal the rudiments of nerves and skin elements. It is 

 difficult, he remarks, to understand the late connection of the midgut with the central nervous 

 system and the scattered mesoderm cells which form the muscles, but he says, in conclusion: 



Bei der Unfce/rscbung der wnnderbaren Eutwicklungsersoheinungen iii tier organischen Well :il>rr driiugt sicli 

 bei tieferer Betrachtuug inimer winder di-r < ii-danke nuf, dass man sohou voni ersten Stadium au eiiicm uutreunliarrn 

 Ganzen gegeuiibersteht. (o4, \>. *<i. ) 



This view of the intimate colonial relations of all the cells of the embryo can not be disputed, 

 since it follows from the common descent of all the somatic cells from the ovum. But the relation 

 which the embryonic cells bear to each other as the un differentiated ectoblast to mesoblast must 

 be of a very different nature from that which exists between muscle fiber and nerve in tiie differ- 

 entiated state. It seems more probable that the union of the central nervous system with other 

 organs by means of nerves is strictly a secondary one, and that the latter arise by budding, as in 

 vertebrates, from the central ganglia. The only observations which I have made on the develop- 

 ment of nerves (see section i) refer to those of the first and second an ten me (PI. LV, Figs. 

 213-U1G, . au., n. a. y.; PI. LVII, Fig. Ii43, n. au.). The anteuuular nerve, which supplies 



