10 MEDULLARY GROOVE. 



The type most fully investigated by Kowalevsky is Ascidia 

 (Phallusia) mammillata ; and the following description must be 

 taken as more especially applying to this type. 



The segmentation is complete and regular. A small seg- 

 mentation cavity appears fairly early, and is surrounded, ac- 

 cording to Kowalevsky, by a single layer of cells, though on 

 this point Kupffer (No. 27) and Giard (No. 11) are at variance 

 with him. 



The segmentation is followed by an invagination of nearly 

 the same character as in Amphioxus. The blastosphere resulting 

 from the segmentation first becomes flattened on one side, and 

 the cells on the flatter side become more columnar (fig. 8 I.). 

 Very shortly a cup-shaped form is assumed, the concavity 

 of which is lined by the more columnar cells. The mouth of the 

 cup or blastopore next becomes narrowed ; while at the same 

 time the embryo becomes oval. The blastopore is situated not 

 quite at a pole of the oval but in a position which subsequent 

 development shews to be on the dorsal side close to the posterior 

 end of the embryo. The long axis 

 of the oval corresponds with the 

 long axis of the embryo. At this 

 stage the embryo consists of two 

 layers ; a columnar hypoblast 

 lining the central cavity or archen- 

 teron, and a thinner epiblastic 

 layer. The dorsal side of the 

 embryo next becomes flattened 



(fig. 8 II.), and the epiblast cover- 



., . . ,. r , , FIG. 5. TRANSVERSE SECTION 



ing it is shortly afterwards marked THROUGH THE FRONT KND OF AN EM- 



by an axial groove continued for- . B fJ J PHALLUSIA MAMMILLATA. 

 (After Kowalevsky.) 



wards from the blastopore to near 



The embryo is slightly younger 

 the front end of the body (fig. 5, than that represented in fig. 8 in. 



mg}. This is the medullary mg , medullary groove; al. ali- 



groove, and it soon becomes con- mentar y tract - 

 verted into a closed canal the medullary or neural canal 

 below the external skin (fig. 6, n.c}. The closure is effected by 

 the folds on each side of the furrow meeting and coalescing 

 dorsally. The original medullary folds fall into one another 

 behind the blastopore. so that the blastopore is situated on the 



