LANCKLET-KISIIKS. 



121; 



lopnieut as in the lii^iicr Nerteliratcs". Ik'twccn tlu^ 

 first two germinal layei's an intiTuiediate layer (tlii' 

 mesoderm, tig. 37"2, A') is fdrnied : and now lliat the 

 intestinal eanal and an iiici|iient aliduniinal ("uity haxc 

 begun to differentiate themselves, there develop a noto- 

 choni and, by means of bulgings and constrictions of 

 the intestinal canal, priiiiitirc rcrtcbire {protorerffltm'), 

 which increase their nnmbci' back\vards from in front. 

 The foremost bulges (tigs. ;-!7,'-) and ;)74) meet with a 

 singular fate. The right [drd) d\vindles into the hol- 

 low at the \\\) of the snout. The left, on the other 



{(/I) on the right side of the intestinal canal becomes 

 a gland'' whi<h also disappears at the close of the lar\al 

 period, and whose outer efferent duct bends roiuid be- 

 low to the left side of the body (fig. 875). But in the 

 median \entral line the intestinal canal coalesces at one 

 point after another, backwards from in front, witli the 

 wall of the body, becomes thickened there (tig. 874, at I), 

 and cNcaitually opens into gill-slits (tig. 875), which are 

 tirst moved up to the right, but afterwards migrate to 

 the left side of the body. In the meantime, however, 

 new gill-slits (tig. 378) have been similarlj' formed in 





Fig. 374. Opficnl section uf » larva with its protovertebrEe, seen from the right. After Hatschek. X 280. 

 Ucrr tlie hiteral dilatations of tht- foreniost part of the intestinal canal have diverged widely; the right (dvd) is elongate and thin-walled, 

 the left (dvs, which from that side shines through in the figure, at about the middle of the former) has thicker walls (deeper cells) and is 

 round. Both separate by constriction from the intestinal canal, the anterior end of which is thereby thrust further back. A glandular growth 

 (the so-called club-shaped gland of Hatschek, gl) has besides begun to develop on the inside of the intestinal canal by means of a trans- 

 versal constriction, first canaliculate and afterwards, owing to the coalescence of the edges of the groove, tubiform, of the right side of the in- 

 testinal wall behind the said lateral dictation. Behind this gland the wall of the intestinal canal has been thickened in the inferior (ventral) 

 median line of the larva and has coalesced with the ectoderm at a point (/) where the first gill-opening afterwards appears. As ye( there 

 is no true mouth; and the intestinal canal is continuous behind with the cerebrospinal. 



Fig. .175. .\ larva 1' , mm. Umg, seen from the left. After Ray Lankestek and Willey. 

 Of the four gill-slits, which are situated on the right side (/ — IV), the foremost three are rather large, the fourth is rudimentary. The 

 first is visible through the posterior part of tlie semioval mouth aperture, which belongs to the left side of the body. Through the anterior 

 part of this aperture are visible, .also from the right side of the body, both the club-shaped gland (j/l), the outer orifice of which may be 

 seen below, on the wall of the body below the mouth aperture, and, just in front of the said gland, a transversal, but curved thickening (e/irf), 

 bisected along the middle by a groove, the future endostyle. Before the upper edge of the mouth aperture appears the opening of the funnel- 

 shaped, ciliated organ (cil). Above this and from the anterior to the posterior end of the body (in front of the caudal fin) lies the notochord 

 (ch), divided into its numerous transverse disks. Above the notochord, and posteriorly round the termination thereof, the spinal cord (m) is 

 extended, with the large pigment spot (ocular rudiment, oc) at its anterior end and some smaller sparse pigment spots on its sides. In the 

 lanceolate caudal fin a sparse collection of pigment spots is also visible. The number of the myomeres is 36; the first myocomma runs 

 obliquely across the anterior edge of the ocular rudiment and down along the subjacent part of the notochord. In the posterior half of the 

 intestinal canal (below the 15th and I6th myomeres) the wall of this canal is thickened (v), an indication of the rudimentary stomach. 



The anal aperture (oh) lies on the left side of the body. 



hand, develops into a funnel-shaped, ciliated organ, 

 probably an organ of smell, which disappears, however, 

 at the termination of the larval period. A special bulge 



a row higher up on the right side, and these become 

 the permanent gill-slits on that side. This only ap- 

 plies, however, to the anterior part of the branchial 



" Branchiostoma differs, liowcver, from the higher vertebrates in that the rudiment of the spinal cord 

 jacent cells of the ectoderm before the canal is closed above (Hatschek). 



'' See Wit.LEY, Quart. Journ. Micr. Sc., n. ser., vol. XXXII, pt 2 (Xo. CXWI. March 1891). p. 209. 



separated from the circuni- 



