T 9 4 COMPARATIVE ANATOMY 



epiblast from behind forwards, the neural groove opens in 

 front by an aperture called the neuropore. This aperture 

 is gradually shifted forwards, till it lies near the anterior end 

 of the embryo ; but it is not closed till the adult characters 

 are fully established, and traces of it are always to be found 

 in the cord of cells uniting the bottom of the olfactory pit 

 with the nerve cord. 



Meanwhile, important changes have been taking place in 

 the roof of the archenteron, leading to the formation of the 

 notochord and mesoblastic somites. The roof of the arch- 

 enteron is composed of smaller cells than those forming the 

 floor and sides. Partly, it would seem, because of the pressure 

 of the neural plate above, but partly as a result of its own 

 growth, the archenteric roof is furrowed by three parallel, 

 longitudinal grooves, one median and two lateral, the latter 

 being the first to appear, and the deepest. As may be seen in 

 the figures, the median groove gradually becomes constricted 

 off from the archenteron from before backwards, the cells 

 composing its walls grow towards and interdigitate with one 

 another, and eventually they become vacuolated, and take 

 on the characters of notochordal tissue. The notochord, 

 then, is formed from a groove in the dorsal wall of the arch- 

 enteron. At first it does not extend to the anterior end of the 

 body, but at a later stage it grows forward till it projects 

 beyond the first myotome. The posterior end of the noto- 

 chord is for a long time confluent with the hypoblast in front 

 of the neurenteric canal. 



The lateral grooves give rise to the third and succeeding 

 pairs of ccelomic sacs or mesoblastic somites in the following 

 manner : The anterior end of each groove is constricted off 

 both from the archenteron and from the remainder of the 

 groove behind it, and so forms a small sac or somite contain- 

 ing a cavity which was originally a part of the archenteric 

 cavity. By a series of successive constrictions, new pairs of 

 somites are constricted off from the lateral grooves, the latter 

 increasing in length, pari passu, with the increase in length of 

 the embryo. The result is the establishment of a number of 

 pairs of somites, derived from the dorso-lateral walls of the 

 archenteron, containing ccelomic cavities derived from the 

 archenteric cavity. These cavities may be obliterated for a 

 time, but soon reappear. When the ccelom is derived in this 



