258 



FiR. (1. I'jiihryo, 12 hours old. showing a slightly more advanced stage in the formation 

 of the ainnlotie iiivagiiinlion and a beginning ditVercnliation of the hydroeoel. 



7. lindjryo, ;i(l hours old, showing the arehenleron completely separated oil from the 

 blastoporus. 



8. Miid)ryo, '2-' , days old, showing a further advanced stage of the amTiiolic invagina- 

 tion, which has formed a furrow nearly round Ihc body. A distinct, broad ciliated 

 band is seen round the middle (d' the body. The archenteron has been separated 

 oil from the blastoporus. TJie internal structures very indistinct in this specimen. 



it. lunbryo. ;{■'/, days old. The tubcfcct of the young urcliin liave begun to jjrotrude 

 through the amniotic invagination. The internal structures not to be made out 

 in this sjjecimen. 



- 1(1. I'"ml)ryo, ;J^/, days old. The primary lubefeet and the lirst spines (mind)ering U), 



both together, in this specimen) are protruding through the amniotic cavity, now 

 split open. The pigment shows a rather distinct arrangement in bands. 



- II. l-jnbryo, 2^ , days old, showing the five [irimary tubefcet lying within the amnion. 



- 12. l-jubryo, J' ., days old. The primary lubefeet and spines have further protruded, 



and the oral and aboral end of the larval body are pushed uj) on the dorsal side of 

 the urdiin. 



- 13. Nearly metamorphosed sea-urcliin; Ihc al)oral part of the larval body has been 



nearly completely absorbed, forndng only a small prominence on the upper side. 

 The primary tubcfeet arc fully extended. lMid)ryonal spines have begun to appear 

 also on the aboral side. 



- 1 1. Ivmbryo, 4V2 days old. The amniotic invagination has been widened so much that 



the primary tubefeet and the spines are assuming a circular arrangement. Only 

 two of the tubefeet extended. The pigment forms a very prominent band above 

 the widening of the body. 



Figures 1- i) and 11 are drawn from specimens mounted in balsan) and thus 

 made transparent; figures 10 and 12 — 14 are drawn trom specimens not thus clear- 

 ed UJ). 



Plate XVIII. 



All ligurcs of Jlcliocidaris cryllirogramina (Val.). All, except lig. 1, '*"/i. 

 big. 1. Section of an embryo, 6 hours old, in a young cleavage stage. The formation of 

 the mesenchyme has begun. The nuclei very indistinct. 



2. Section of an embryo, IS hours old, in a young blastula stage. The whole of the 

 blastocoel cavity filled by mesenchyme cells, still in an embryonal condition. 



'.i. Section of an embryo, 18 hours old, in the fully formed blastula stage. The mesen- 

 chyme cells have assumed the character of a reticulum, interwoven between the 

 large vacuoles, containing a probably fatty substance which serves as nourishment 

 to the developing embryo. 



■1. Part of a longitudinal section of an embryo, 2-' ., days old. showing the furniation 

 of the amniotic invagination. -»7i- 



5. Longitudinal section of an embryo, 2'/, days old: showing the primary tubefeet (t) 

 protruding into the amniotic cavity (am.), which opens out through a small pore, 

 c. enlerocoel. 



6. Part of a section from the same series as fig. o. The amnion is closed here, 

 h. hydroeoel. 



7. Longitudinal section of an embryo, 42 hours old. Showing the hydroeoel (h) lying 

 close towards the amniotic invagination (am), which is here still an open groove. 

 Trace of the blastoporus is seen in the oral end; ent. entoderm. 



8 — 0. Two longitu<linal sections of an end)ryo in metamorphosis, 4'/., days old. In 

 lig. 8 is seen the stone canal and its outer ojjening, in lig. SI the inner opening of the 

 stone canal into the hydroeoel. a. ampulla; c. coelom; tl. dorsal: ent. entoderm, 

 h. hvdrocoel; r. reservoir of food ; st. c. stone canal: v. ventral. 



