442 MEMOIRS OF THE NATIONAL ACADEMY OF SCIENCES. 



blood cells have penetrated, and here eventually the sternal artery is developed. While the evi- 

 dence is not conclusive, we have only to decide between the former conclusion — that the intrusive 

 tissue is derived from the wandering cells, and is to be referred to mesoblast, or the view that it 

 represents differentiated ectoblast. 



A general account of the structure of the nervous system of the larva is given in the first 

 section. Further than this the details of development have not been followed. 



In comparing this account with that given by Keichenbach for the crayfish, Astacus fluviatilifi, 

 there are numerous particulars in which there is no agreement, while in some important matters 

 Te are in accord. 



I find in Alpheus the oral invagination occurring on a line drawn between the bases of the 

 antennular buds, and I have a great many preparations of the eggs of the lobster. Homarus anieri- 

 canuH, which show the earliest traces of the stomodteum. Before th?. first antenniB are folded, 

 when they are distinguished as dense patches of cells, some eggs show the primitive mouth as a 

 minute circular pit, lying nearly on a line drawn between the centers of these proliferating cell 

 areas, but, so far as my observation goes, never distinctly in front of them. 



The-relative positions of the mouth and first pair of antennae shift very rapidly during the 

 early period of their growth, which precedes the fully developed egguaui)lius condition. The 

 pit elongates and becomes a transverse furrow, and by the time the first pair of auteniiie are 

 clearly marked off" as rounded buds, and before the second pair are raised into folds, the mouth is 

 still on a line with the first of these appendages. When the second anteuuie are elevated into 

 folds the mouth is behind the buds of the first pair, or on a line between their posterior edges. 



Keichenbach (taf. ii, Fig. 7a, Lb.) describes and figures a cell thickening between the "Kop- 

 flappen" of the crayfish embryo (Stage B), which he considers the beginning of the labrum. His " 

 sections show that below this point a mass of ectodermic cells occurs, which is interpreted as the 

 "Vordarmkeim." The mouth is not represented as appearing until the following egg-nauplius 

 stage (Stage F. Corai)are Fig. 66 and p. 100, § 7, " Der Vorderdarm"), when it occupies a j)osition 

 exactly comparable with that observed in the lobster. I therefore can not agree with Kingsley in 

 saying that Keichenbach "has all the appendages at first distinctly postoral." While the posi- 

 tion of the Crustacean appendages may have been primitively postoral, it may be questioned if 

 in the higher Crustacea the first antennre ever arise behind the mouth invagination. 



Kingsley describes the position of the mouth in Crangon as distinctly postoral, but an inspec- 

 tion of figure 11 of his paper, leaves some doubt as to whether he is not mistaken in this particular. 

 A single rudimentary appendage, marked as the first antenna, is represented as occupying nearly 

 the entire space between the optic disk and thoracic abdominal fold. This does not agree with 

 my own preparations,* and since in Alpheus, Homarus, and Astacus the mouth does not appear 

 until a dense stratum of cells carpets the intermediate space between the optic disks and lateral 

 cords, there is some difticulty in interpreting the cluster of cells marked »», in Kingsley's paper, as 

 the invagination of the mouth. t 



In Alpheus, Homarus, Astacus, and probably in Decapods generally, the ganglion of the first 

 antenna cannot be said to be postoral, but its development begins nearly on a line with the 

 invagination of the pi-imitive mouth. The ganglion of the second antenna is developed behind 

 the primitive mouth, but gradually shifts forward with its appendage until it comes to lie, in the 

 larva, considerably in front of the mouth. In this movement, the ganglion however outstrips the 

 appendage. The ganglia of these two appendages unite to form the brain or supra oesophageal 

 ganglion. The ganglion of the first pair of antennfe is constricted into two portions marked by 

 an oblique, transverse line at the surface. The anterior of these parts Keichenbach calls the 



* I have preparations of the eggs of Crant/on vulgaris, in various stages of development, from ttie segmentation 

 onward. In one egg, which is somewhat more advanced than that of figure 10 (SI), or than the Alpheus in Fig. M 

 of this paper, the optic disks and ventral pl>ate are dense patches of cells. On either side of the ventral plate and in 

 close relation to it there is a marked area of cell proliferation which represents the mandible. In the sp.ace between 

 this and the antenna the nuclei are more scattered, but the kaiyokinetic figures show the activity of cell division. 

 In a late nanplius stage the stomodiBum is on the middle line between the first and second antennae, and the anten- 

 nular ganglion is segmented into two parts on each side, as shown for Alpheus in Fig. 110. 



tThis criticism is supported by Woldon's observ.ations on Crangon, who, with reference to this subject, says: 

 "The first antennie .are evidently prteoral from the very earliest period at which the mouth is visible." Op. cit. 



