Ud viklingens videre tbrlob er mi i hovedtrækkene føl- 

 gende. Medullarpladeu brever sig bagtil og til siderne af 

 gastrulamunden i form af medullarvulsten, der forst vokser 

 over gastrulamunden (Fig. 3) og saa videre fremover mod 

 gastrulaens forende, idet samtidig gastrulamunden r3-kker 

 indad (ventralt og bagtil) Fig. 4, 5. Medullarroret forbliver 

 berunder stadig aalient fortil og fortsætter sig bagtil i ga- 

 strulamunden, canalis neurentericus. 



Det nævnte fælles anlæg for mesoderm og chorda (Fig. 

 3) deler sig nu i r.iesodermcellerne lateralt og cborda i 

 midten. Ciiorda rykker endvidere stadig længer bagover 

 idet samtidig larvens liagerste kropsparti smalner sterkt af 

 til dannelse af baien (Fig. 4 og 5). Entodermen begrænd- 

 ser herved væsentlig til det forreste kropsparti, bvorfra 

 siden bele tarmtractus og gjælletarmen dannes, medens halen 

 af de oprindelige entodermelementer kun indeholder anlæg- 

 get til cborda, og en encellet tynd ventral entodermstreng 

 (eii) (for ikke at nævne mesodermelementerne, der danner 

 luilens muskulatur). 



Efter denne beskrivelse vil ogsaa det efter r. Dari- 

 doff afbildede stadium af Disfaplin magnilarva. Pl. XII, 

 fig. 1. være foi-staaeligt. Man iagttager ber dorsalt medul- 

 larroret, fra hvis bagre parti en tildels lukket canalis neu- 

 rentericus gaar over i den tlercellede entoderm, anlægget 

 til baien, cborda o. s. v. 



Vi vil da efter denne skitse kunne omtale organdan- 

 nelsen bos larven og liegynder med ceniralnervesystemet. 



Paa Plancbc XII findes afbildet en række stadier af 

 nervesystemets udviklir.g bos Disfaplia magnilarva. De 

 forste stadiei', der paa det omhyggeligste er undersogt af 

 v. Daridoff, bar jeg derfor ingen fornyet undersogelser un- 

 derkastet, saa meget mere som v. Davidoffs resultater 

 stemmer paa det bedste overens med undersogelser hos 

 andre grupper. Man iagttager paa v. Davidoffs figur, Pl. 

 XII, fig. 2, at medullarroret lukker sig fortil og derved 

 lielt losrives fra ektodermen. Det er altsaa paa dette sta- 

 dium et dorsalt ror beliggende i larvens længdeakse mellem 

 ektoderm og entoderm. Bagtil bænger det endnu sammen 

 med den caudale fortsættelse af tarmtractus, canalis neu- 

 rentericus. Et noget ældre stadium seet fra dorsalsiden 

 (ovenifra) er efter egne undersogelser afl^ildet Pl. XII, fig. 

 3. Man gjenkjender her medullai'roret, der er beliggende 

 mellem de to anlæg til periln-ancbialsækken (Ektodermen 

 kunde ikke indtegnes og maa tænkcs fjernet). Medullar- 

 roret viser lier et bestemt begrændset forreste parti, anlæg- 

 get til ,.bjerneblæren-', „sandseblæren-', ,,larvebjernen", der 

 allerede viser sig svagt antydet paa fig. 2, og et bagre 

 parti, et fint ror. Hjerneblæren viser paa sin hoire side 

 en tydelig udbugtning, af hvilken siden den egentlige larve- 

 bjerne fremgaar. Den venstre væg viser en svag fortyk- 

 kelse, den forste antydning til det „l)livende"', ,.definitive 

 ganglion-', der, som bekjendt, udgjor ccntralnervesysteniet 



The principal features in the furtber course of deve- 

 lojnnent are as follows. The medullary plate rises behind 

 and at the sides of the gastrula aperture in the form of 

 medullary ridges, which first grow over the gastrula aper- 

 ture (fig. 3), and then forwards towards the auterior end 

 of the gastrula, tiie gastrula aperture at the same time 

 moving inwards (ventrally and backwards) (figs. 4, 5). The 

 medullary tube meanwbile remains open in front, and is 

 continued backwards in the gastrula aperture, the neuren- 

 toric canal. 



Tiie before-mentioned common rudiment of the meso- 

 derm and the notocbord (fig 3) is now divided laterally 

 into mesoderm cells, witb the notocbord in the middle. 

 Tlie latter continues to move fartber back, wbile at the 

 same time tiie bindmost portion of the larvas body tapers 

 rapidly to form the tail (figs. 4, 5). The endoderm is thus 

 virtually confined to the fore part of the body, from which 

 the whole alimentary canal, and the branchial gut are 

 su1)sequently farmed, while, of the original endoderm ele- 

 ments the tail only contains tiie rudiment of tiie notocbord 

 and a thin, simple-celled, ventral endodermal cord (en), 

 not to mention the mesoderm elements tbat form the mus- 

 culature of the tail. 



After this description, v. Davidofs Diduplia magni- 

 larva stage, shown on Pl. XII, fig. 1, will also lie intelli- 

 gible. Here the medullary canal is seen dorsally, a par- 

 tially-closed neurentei-ic canal passing from its bindei- jiart 

 into the many-celled endoderm, the rudiment of tlie tail, 

 notocbord, etc. 



After this sketch, we shall be able to descrilje the 

 formation of the organs in the larva, lieginning witii tlie 

 central organs of the nervous system. 



On Pl. XII will be found represented a series of 

 stages in the development of the nervous system in Di^fa- 

 plia magnilarva. As the earlier stages have been most 

 carefully observed loy v. Davidoff'. I liave not subjected 

 tbem to any renewed investigation, particularly as v. Davi- 

 doffs results agree perfectly witii investigations in otber 

 groups. It will be observed in v. Davidoff"s figure, Pl. 

 XII, fig. 2, tbat the medullary canal is closed anteriorly, 

 and is thus completely detacbed from the ectoderni. There 

 is also, at this stage, a dorsal tul)e situated in the great 

 axis of the larva, between the ectoderni and tlie endoderm. 

 Posteriorly, it is still attacbed to the caudal continuation 

 of the intestine, the neurenteric canal. On Pl. XII, fig. 

 3, a ratber more advanced stage, seen from the dorsal side 

 (above), is represented after personal observations. Here 

 the medullary canal is recognisal)le, situated lietween the 

 two rudiments of the peribranchial cavity. (The ectoderni 

 could not lie included, and must be considered as removed). 

 The medullary canal here shows a distinctly defined an- 

 terior part, the rudiment of the ,,cerebral vesicle", the 

 ..sensory vesicle-' or the ..larval brain", which is already 

 faintly iudicated in fig. 2; and a posterior part, a tiiin 

 tube. The cereln-al vesicle exhibits, on its right side, a 

 distinct protuberance, trom which the true larval brain sub- 

 sequently makes its appearance. The left wall shows a 



