380 



EMBRYOLOGY 



the result of it, which is at first spherical, but subsequently 

 becomes flattened on one side. On this side an invagination 

 then takes place, and the result is a typical invagination 

 gastrula. Soon, however, the originally wide blastopore 

 contracts to a short, narrow fissure. At the same time the 

 external surface of the embryo becomes covered with short 

 cilia, and a circle of stouter cilia makes its appearance in 

 the vicinity of the blastopore. Subsequently the blastopore 

 entirely closes. The two primary germ-layers remain 

 nnited at this point, but only by a plug of cells ; finally, they 



separate entirely from each 

 other, so that the embryo 

 then consists of two sepa- 

 rate cell-vesicles lying one 

 within the other. At the 

 same time the embryo 

 elongates somewhat and 

 then assumes a shape sucb 

 as is represented in Fig. 

 168 ^4. At about this stage 

 or even somewhat earlier, 

 the embryo breaks through 

 the egg-membrane and be- 

 comes a free larva, which 

 does not, however, lead a 

 pelagic existence, but lives 

 on the bottom and is found 

 in places where the water 

 is not very deep. 



The internal structure of 

 the larva is soon changed 

 in such a way that an in- 

 ternal segmentation can be recognized. The elongated, com- 

 pletely closed archenteron bulges out at its anterior end, 

 and forms a pair of diverticula, which are directed back- 

 wards (Fig. 167 Ci). These are constricted off from the 

 archenteron in connection with each other, and lie in front 

 of it as closed vesicles. In the same way two pairs of 

 coelomic sacs are formed further back as evasrinations of 



Fig. 167.— Diagram of a longitudinal 

 section through a larva of Balanoglossus 

 Kowalevskii{fitter Bateson). c,, anterior, 

 t'ri, middle, Cm, posterior, coelomic sacs ; 

 d, iutestine. 



