Emhryology of the River- Lamprey. ^5 



After this the chief centre of the developmental processes is 

 situated for a time in the hinder portion of the embryo. In 

 successful longitudinal sections, namelj such as pass through 

 the middle of tiie ovum from front to rear, so that they bisect 

 the upper and lower lips of the anus of Rusconi, it can be 

 seen that the embryo already extends over half of the ovum. 

 It consists, as already stated, of ectoderm and endoderm, and 

 each of these layers exhibits only a single series of cells 

 throughout its entire extent. It is true that sections are to 

 be found in which the ectoderm has more than one series of 

 cells, but this is owing to the fact that the sections were taken 

 obliquely and that the knife, working more on the surface of 

 the ovum, has carried a larger strip of this with it. I have 

 found preparations in which the enteric cleft communi- 

 cated with the segmentation-cavity. The cells which were 

 situated at the periphery of the cavity, immediately beneath 

 the ectoderm, were utilized for the formation of the lower 

 layer. 



Ova 118 hours after Fertilization. 



Ova belonging to this period exhibit but little difference 

 from those of the preceding one. They have become some- 

 what longer; tlie two lips, the upper and the lower, and the 

 plug which lies between them are still more distinct, and the 

 embryo itself has still further increased in length. In many 

 longitudinal sections each of the two layers continues to exhibit 

 a single layer of cells. On the other hand, many transverse 

 sections, if they have passed through the posterior end of the 

 ovum, exhibit a multiplication of the ectoderm cells beneath 

 the dorsal furrow. The spinal cord is in process of forma- 

 tion : as Calberla has already shown, this structure is at first 

 solid, and it is not until later that the cells separate from one 

 another and give rise to the spinal canal. 



Ova 126 hours after Fertilization. 



This is one of the most interesting periods. The embryo 

 has become considerably longer. In very many transverse 

 sections of the ovum the embryo is cut through in two places, 

 at the anterior and posterior end. At the latter we observe 

 the solid spinal cord, beneath which is the notochord already 

 completely developed, and then the epithelium of the enteric 

 canal, which forms its inner wall. On both sides of the 

 notochord lie in part still solid rudiments of the protover- 

 tebrge, Wolffian ducts, and lateral plates ; the latter pass into 



8* 



