322 



FORMATION OF THE LAYERS. 



believed by Hatschek to assist in absorbing the albuminous fluid in which 

 the eggs are suspended. These large cells are eventually covered by the 

 normal epiblast cells and subsequently disappear. In both these types 

 the hypoblast cells undergo, during their invagination, peculiar changes 

 connected with their nutritive function. 



In Euaxes (Kowalevsky) the segmentation is far more unequal than in 

 the other types ; a typical epibolic invagination takes place (fig. 140), and 

 the blastopore closes completely along the ventral surface. 



In all the oligochaetous types, with the exception of Euaxes, 

 where the blastopore closes completely, the blastopore becomes, 

 or coincides with the mouth. In 

 Serpula it is stated (Stossich), 

 as we have seen, to coincide 

 with the anus : a statement 

 which receives confirmation 

 from the similar statements of 

 Willemoes-Suhm (No. 358). It 

 is necessary either to suppose 

 a mistake on the part of Stos- 

 sich, or that we have in Chaeto- 



pods a case like that of Gas- 

 teropods in which a slit-like 



FIG. 140. TRANSVERSE SECTION 

 THROUGH THE OVUM OF EUAXES 

 DURING AN EARLY STAGE OF DEVELOP- 

 MENT. (After Kowalevsky.) 



ep. epiblast ; ms. mesoblastic band ; 



blastopore originally extending } >y- hypoblast. 



along the ventral surface may in some forms become reduced 



to a pore at the oral, or in other forms at the anal extremity. 



So far only two germinal layers the epiblast and the hypo- 

 blast have been spoken of. Before the invagination of the 

 hypoblast is completed the mesoblast makes its appearance in 

 the form of two bands or streaks, extending longitudinally for 

 the whole length of the embryo. These are usually spoken 

 of as germinal streaks, but to avoid the ambiguity of this term 

 they will be spoken of as mesoblastic bands. 



Their origin and growth has been most fully studied by 

 Kleinenberg (No. 341) in Lum. trapezoides. They commence 

 in this species shortly before the gastrula stage as two large 

 cells on the surface of the blastoderm, which may be called 

 mesoblasts. These cells lie one on each side of the median 

 line at the hind end of the embryo. They soon travel inwards 

 and become covered by the epiblast (fig. 141 A, ;'), while on 

 their inner and anterior side a row of small cells appears (ms). 



