34 ZOOLOGY 



medullary groove. Laterally certain cells of the endoderm, in close 

 relation to those forming the rudiment of the notochord, divide to 

 give rise to a pair of longitudinal strands of cells the rudiments 

 of the mesoderm (Fig. 743, mes.). During this process of mesoderm- 

 formation, there are no diverticula developed from the archenteron. 



The embryo (Fig. 743, B) now becomes pear-shaped, the narrow 

 part being the rudiment of the future tail. As this narrow portion 

 elongates, the part of the enteric cavity which it contains soon 

 disappears, coming to be represented only by a strand of endoderm 

 cells, which gives rise in the middle to the extension backwards 

 of the notochord, laterally to the mesoderm of the tail, and ventrally 

 to a cord of endoderm cells continuous with the wall of the enteric 

 cavity in front. 



The caudal region increases in length rapidly, and the anterior 

 or trunk region, at first round, becomes oval. At its anterior end 

 there appear three processes of the ectoderm, the rudiments of the 

 adhesive papillce (Fig. 744, adh.), organs by which the larva sub- 

 sequently becomes fixed. The ectoderm cells at an early stage 

 secrete the rudiments of the cellulose test ; in the caudal region 

 this forms longitudinal dorsal and ventral flaps having the function 

 of unpaired fins. 



The medullary canal becomes enlarged at its anterior end. A 

 vesicular outgrowth from this enlarged anterior portion forms the 

 sense-vesicle (sens, ves.) The posterior narrow part forms the caudal 

 portion of the central nervous system (spinal cord). Masses of 

 pigment in relation to the sense-vesicle early form the rudiment 

 of the two larval sense-organs, otocyst (or statocyst) and eye. 

 The part behind this presents a thickened wall with a narrow 

 lumen. This is known as the ganglion of the trunk. The rudiment 

 of the neural gland early appears on the ventral wall of a ciliated 

 diverticulum (oil. gr.) of the anterior end of the archenteron (future 

 pharynx), which subsequently unites with an outgrowth from the 

 medullary canal. 



The embryonic alimentary canal consists of two regions, a wide 

 region situated altogether in front of the notochord, and a nar- 

 rower portion situated behind in the region of the notochord. The 

 wider anterior part gives rise to the pharynx, the posterior part 

 to the oesophagus, stomach, and intestine. The mouth-opening is 

 formed shortly before the escape of the embryo from the egg ; an 

 ectodermal invagination is formed at the anterior end, and an 

 endodermal diverticulum from the archenteron grows out to meet 

 it ; the two coalesce, and the oral passage is thus formed. 



The rudiments of the heart and pericardial cavity first appear 

 as a hollow outgrowth from the archenteron : this subsequently 

 becomes constricted off and involuted to form a double- walled sac, 

 the inner layer of which forms the wall of the heart, while the 

 outer gives rise to the wall of the pericardium. 



