No. I.] DEVELOPMENT OF THE COMPOUND EYE. 53 



anterior extension of the mesoderm, and at the left side it may- 

 be seen extending itself down between the cephalic ganglion and 

 the gangliogenic tissues towards the optic cavity. Later sec- 

 tions show that it then turns upwards, grows into the cavity, 

 but retains its primitive character and its single-celled thickness 

 until after the young shrimp escapes from the egg. I have not 

 fully traced its later stages, but have evidence to show that in 

 the adult it forms the thick layer of pigmented connective- 

 tissue, which sheathes the nerve-fibres between the ommatidia 

 and the outer ganglion of the eye-stalk of the adult. 



The next series of changes are shown in Fig. 7, which repre- 

 sents a portion of the eye of an embryo much further 

 advanced. Only seven appendages are as yet outlined ; but these 

 show plainly the distinctions between maxillae and maxilHpeds. 

 The abdomen is cylindrical, and is terminated by a bifurcated 

 telson armed with the typical fourteen spines ; the ganglia of 

 the abdomen are outlined, the heart is formed, and has begun to 

 beat, and the deposition of pigment has begun in the eye, though 

 this is not shown in the figure under discussion. 



First it is to be noticed that the ectodermal nuclei have 

 increased in number, and have come to correspond in position 

 with those of the underlying layer. The nuclei of the retinogen 

 which, in the stage last described, were elongate, have divided 

 just as did those of the gangliogen, and each has given rise to 

 five nuclei arranged in a row. These rows are arranged in sets. 

 In the sections two will be seen closely appressed to each other, 

 and separated from the adjacent pairs by a rod of apparently 

 structureless material. As subsequent development shows, this 

 intervening substance is the rudiment of the crystalline cone, and 

 the adjacent rows of nuclei in reality belong to different omma- 

 tidia or optic elements. Horizontal sections show that there are 

 four of these rows to each ommatidium, but only two of these 

 can be shown in one section. The optic cavity retains its early- 

 shape, and the mesoderm (exaggerated in thickness in the draw- 

 ing) shows no change. In the ganglionic layer a change is 

 visible. The rows of nuclei have broken in twain, and have 

 formed the rudiments of two ganglia, one retaining four, and 

 the other two of the nuclei of the last stage. Between these 

 two a fibrous area has arisen, in which occasional nuclei, stain- 

 ing less deeply than those of the surrounding parts, can be seen. 



