716 EMBRYOLOGY. 



taken jilace defining the arms more sharply by the indentations of the vibra- 

 tile chord. The intestine, the stomach, and the oesophagus are clearly dis- 

 tinguished by the different character of their walls. The water-tubes are not 

 united, and have not increased in size. This condition presents a material 

 difference in the degree of development when compared with the corre- 

 sponding stage of a starfish (Fig. 11, Proc. Am. Acad.; PL I. and II., Emb. 

 Starfish). Here the water-tubes occupy the most important portion of the 

 embryo, while in the sea-urchin pluteus the most striking characteristic is 

 the amount of room taken up by the stomach and oesophagus compared with 

 that occupied by the water-tubes. 



In the subsequent stages i Figs. 49, 50) the Echinus embryos have reached 

 forms which arc already more familiar to us from the drawings of Muller; 

 they resemble closely some of the figures given by him of Strongylocentrotus 

 lividus in his fourth Memoir. A good deal of allowance must be made for 

 the differences of outline between the figures given here and the drawings 

 of Muller. From the evidence of the drawings themselves, it is plain that 

 nearly all the specimens drawn by him are compressed. I have endeavored 

 to represent these embryos as they appear swimming about; it is by no 

 means an easy task to follow them in their almost unceasing movements with 

 the magnifying power required to introduce the necessary details; but I trust 

 I have succeeded in giving a tolerably accurate idea of their appearance in 

 these outline drawings. In an embryo during the tenth day after fecunda- 

 tion {Fig. ;.</). the most important changes are the increase in length of the 

 arms e, e", and the formation of rudiments of another pair of arms, e"' ; the 

 vibratile epaulettes, v", as Midler has called the peculiar accumulations of 

 vibratile cilia situated between the base of the adjoining arms e , (■'". make 

 their appearance at this stage. It is easy to follow them from their origin, 

 when they are simply a thickening of the vibratile chord v" \ Figs. 49, 50), 

 until they have passed through the successive stages represented in Figs. 

 51,52,54, to attain the great size observed in Fig. 55, when they appear in 

 certain positions as having no connection whatever with the vibratile chord, 

 and as having originated independently of the main chord. Muller had not 

 traced their development, and laid great stress on their presence in distin- 

 guishing the different species of sea-urchin embryos. 



With the development of the arms, the intestine loses its former shape ; it 

 has now assumed the appearance of a large elliptical receiver with thin 

 walls. The stomach is somewhat dumb-bell shaped, and the left water-tube 



