442 MEMOIRS OF THE NATIONAL ACADEMY OF SCIENCES. 



blood cells have penetrated, aud 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, aud is to be referred to inesoblast, 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 lieichenbach for the crayfish, Astacusjliiritttilix, 

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

 ;ve are in accord. 



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

 auteunular buds, aud I have a great many preparations of the eggs of the lobster. Homarus ameri- 

 cnniiii, which show the earliest traces of the stomoda-um. Before th;' first aiitemi; are folded, 

 wheu 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 iu front of them. 



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

 early period of their growth, which precedes the fully developed egg-nauplius condition. The 

 pit elongates and becomes a transverse furrow, aud by the time the first pair of antenna? 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 antenna- are elevated iuto 

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



Iteichenbach (I at'. II, Fig. 7<i, Lb.) describes aud figures a cell thickening between the "Kop- 

 flappeu" of the crayfish embryo (Stage E), which he considers the beginning of the labrniu. His 

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

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

 stage (Stage F. Compare Fig. GO aud p. 100, $ 7, " Der Vorderdarm"), when it occupies a position 

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

 saying that Reichenbach "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 

 iu the higher Crustacea the first antenna; e\er arise behind the mouth invagination. 



Kingsley describes the position <>!' the mouth in Craiigon 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 iu 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 difficulty in interpreting the cluster of cells marked m, in Kingsley's paper, as 

 the invagination of the mouth. t 



In Alpheus, Elomarus, Astacus, and probably in I )ec;ipods 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 primitive 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, iu 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 (esophageal 

 ganglion. The ganglion of the first pair of antenna.', is constricted into two portions marked by 

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



* 1 have preparations of the eggs of Grant/on ralgarit, iu various stages of development, from the segmentation 

 onward. Iu one egg, which i.s somewhat, more advanced than that of figure 10 (:il), or than the Alpheus iu Fig. 58 

 11!' tliis paper, the- optic disks aud ventral plate are dense patches of cells. On either side of the ventral plate aud in 

 close relation to it there is a marked area of cell proliferation which represents the mandible. Iu the space between 

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

 In a late nauplins stage the storuodoeum is on the middle line between the first aud second antenna 1 , and the auteu- 

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



t Tins criticism is supported by Weldon's obierv.-iti'Mis on Oaugou, who, with reference to this subject, says: 

 "The first uiiteniiiu are evidently prawral from the very c.urlirst period at which the mouth is visible." Op. cit. 



