LECTURES ON EMBRYOLOGY 



17 



per and lower surface to this umbrella-like disk \ 

 [Plate HI, fig, C] and there is a tubular part and a 

 swollen portion to the peduncle* As soon as the 

 peripheric part of the umbrella begins to spread, we 

 observe five little tubercles forming underneath 5 

 and into these tubercles we see that the peculiar 

 aspect of the middle one extends. Soon there will 

 be other prominent swellings forming \ but two to 

 each of the former ones ; and next, two more, as 

 seen in Plate IV, fig. A, in which the peduncle is 

 represented from below projected upon the centre 

 of the disc. While this is going on, calcareous 

 nets are formed by the accumulation of crystals in 

 the cells of the germ, At first there are little iso- 

 lated crystals formed as nuclei in the cells ; and 

 then several close together will unite and form a 

 little irregular mass, and they will combine so as 

 to constitute a network of solid substance arrang- 

 ed very regularly. They aggregate first about the 

 prominent tubucles of the lower surface, corres- 

 ponding in position to the five primitive ones 

 [Plate IV, fig. B, page 13]. 



Now the points in which these calcareous de- 

 posites take place are symmetrically arranged 

 [Plate IV, fig. B, p. 13], Next, five alternating with 

 these arise in the intervening spaces, [Plate IV, B, 

 p.13] and another is formed in the centre of the disc. 



All these networks are, however, not formed in 

 the same plane of the animal ; those arranged in 

 fives being deposited below, and the middle one 

 above the central mass of yolk in the periferic lay- 

 er of the germ. 



At this period f the peripheric tubercles of the low- 

 er surface become colored in their centre and the ex- 

 ternal calcareous networks spread over them. The 

 red spots of the tubercles are now very conspicu- 

 ous. When examined under a high magnifying 

 power they appear like little heaps of colored 

 dots, and these are so many cells with colored nu- 

 clei. As peculiar organs, they answer to the rudi- 

 mentary eyes of the perfect star-fishes. 



The calcareous nets which were at first only ten in 

 number, become now gradually more and more 

 numerous, marking out more and more distinctly 

 the rays of the little star-fish which are thus form- 

 ing, new being interposed in pairs between those 

 already existing, and small spines projecting from 

 the older ones. (Plate X., A.) 



The tubercles of the lower surface, which alter- 

 nate with them, growing more prominent and 

 elongated, are finally transformed into suckers, as 

 I will call them, or the so called ambulacral tubes, 

 [Plate IV, fig. C.] With the addition of new cal- 

 careous nets they also become more numerous 

 and form finally rows of tentacles, D, E, F. Other 

 changes have also taken place. The cells within 

 the peduncle have undergone changes. Some 

 have become movable, and a kind of circula- 

 tion is going on in them. The internal space along 

 each ray has become more transparent ; the am- 

 bulacral tubes have become hollow, and from that 

 time there seems to be a communication between 



the external water and the internal structure 



* 



What remains of the yolk is more distinctly cir- 

 cumscribed in the centre of the animal, extending* 

 as a star shaped disc into the rays, The radial 

 portion becomes finally distinct from the central 

 one, and we have at last an internal cavity, which 

 is the stomach, from which the coecal appendages 

 of the rays, with their liver-like organ, will be de- 

 veloped [Plate IV, fig. E. p. 13]. The peduncle is 

 reduced to a mere vesicle ; a hole is formed in the 

 centre of the lower surface, the mouth, around 

 which A circular thread becomes visible, answer- 

 ing to the nervous system, and from which other 

 threads extend towards the extremity of the rays,- 

 being the radiating nerves which establish a con- 

 nection between the peripherical colored spots, 

 which are the eyes, and the central nervous sys- 

 tem which encircles the mouth. Before, the young 

 star-fish had thus assumed a life of about one line" 

 in diameter; it has now assumed the form and 

 structure of the perfect animal, To this growth 

 there is one point of peculiar interest I mean the 

 correspondence between the development of the 

 calcareous net works [Pi. IV, fig. B, p. 13, and PI. 

 X, fig. A,] and the arrangement of the solid plates 

 in Crinoids [PI. I, fig. A, p. 13, PI. VII, fig. A, I> 

 p. 14, and Plate X, fig. B.] 



[PLATE X.J 



But I see that the time has past, and I am obliged 

 to conclude. Let me only add a few remarks be- 

 fore I close. The mode of growth in the starfishes 

 as I have illustrated it, does not agree with obser- 

 vations which have been recently made by other 

 investigators. Von Baer, Johannes Muller, and 

 several other investigators, have traced the growth 

 of these animals recently. But they have traced 

 them at another epoch than the development 

 which I have observed here [PI. IIIp.13] ; and it is 

 now probable that in the Echinodewns, also, there 

 are two modes of reproduction during which the 

 growth of the germ is not identical, as in the ani- 

 mals reproducing by alternate generations. It was 

 during summer that the investigators just men- 

 tioned made their observations, and they found 

 that all their germs were surrounded with a most 

 remarkable external frame-work, whilst mine, 

 which are entirely destitute of such envelopes^ 

 were observed growing during winter, at a season 

 when animals in general do not reproduce them- 

 selves. 



However, it is remarkable how many of the low- 



