OH. LIX.] 



THE MESOBLASTIC SOMITES 



833 



vpl 



ridge of epiblastic cells, and soon after its formation a layer of cells, 

 the mesoblast, or third layer of the blastoderm, grows out from 

 its sides and posterior end, and extends 

 between the epiblast and hypoblast over 

 the whole area of the vesicle. 



That portion of the mesdblast which 

 lies immediately at the sides of the neural 

 groove becomes partially separated from 

 the rest, and at the same time divided 

 into cuboidal blocks, the protovertebrae or 

 mesoblastic somites. The more laterally 

 situated part of the mesoblast constitutes 

 the lateral plates, and the narrow strand 

 of mesoblastic cells which connects the 

 lateral plate on each side with the pro- 

 tovertebral somites is the intermediate cell 

 mass. Soon after its formation the lateral 

 mesoblast is cleft into two layers, and the 

 space which appears between the two 

 layers is called the coalom (figs. 627, 628). 

 The outer or somatic layer of the meso- 

 blast adheres to the epiblast; the two 

 together form the somatopleur. The inner 

 or splanchnic layer fuses with the hypo- 

 blast to form the splanchnopleur. Cavities 

 also appear in the mesoblastic somites. 



Whilst the mesoblast is extending and 

 cleaving, the neural folds gradually grow 

 in height, and their free margins turn 

 inwards and fuse together. This fusion 

 commences in the cervical region, and 

 extends forwards and backwards, and when 

 it is completed the neural groove is con- 

 verted into a closed tube, the neural tube, 

 and the original groove is now the central 

 canal of the nervous system. In the ovum 

 at this period there are, therefore, three 

 cavities : (1) The neural or central canal 

 confined to the embryonic region ; (2) The 

 coelom or space in the mesoblast ; (3) The 

 archenteron within the hypoblast The 

 embryonic area is still outspread on the 

 surface of the ovum. When the changes 

 to which reference has been made are well advanced, and in many 

 cases before the neural groove is closed, the embryonic area begins 



pr 



Fio. 629. Embryo chick (36 hours), 

 viewed from beneath as a trans- 

 parent object (magnified), pi, Out- 

 line of pellucid area ; FB, fore-brain, 

 or first cerebral vesicle: from its 

 sides project op, the optic vesicles ; 

 SO, backward limit of somatopleur 

 fold, "tucked in" under head; 

 a, head-fold of true amnion ; a', re- 

 flected layer of amnion, sometimes 

 termed " false amnion " ; sp, back- 

 ward limit of splanchnopleur folds, 

 along which run the omphalo- 

 mesenteric veins uniting to form 

 h, the heart, which is continued 

 forwards into ba, the bulbus arte- 

 riosus ; d, the fore-gut, lying behind 

 the heart, and having a wide cres- 

 centic opening between the splanch- 

 nopleur folds ; HB, hind-brain ; 

 MB, mid-brain ; pv, protovertebra; 

 lying behind the fore- gut ; me, line 

 of junction of medullary folds and 

 ofnotochord; ch, front end of noto- 

 chord ; vpl, vertebral plates ; pr, 

 the primitive groove at its caudal 

 end. (Foster and Balfour.) 



