MESODERM AND THE TAIL IN PETROMYZON. 



57 



•crooked like a hook as in a leech. The spiral winding of the body shown 

 m later intrachorion life is accordingly caused very little by the unequal 

 anteroposterior growth of the dorsal and ventral organs, but is due largely 

 to the insufficient space enclosed by the chorion. 



It is only on the fourteenth day, that the enteric canal formed of the 

 ■enteroderm develops so as to be reduced into a slender tube and becomes 

 equal in length to the dorsal series of organs. The body is now stretched 

 out, and its axis straightened, while the rudiment of the tail no longer 

 looks downwards, but projects posteriorly along the main axis of the body 

 and commences to grow. Only the hindmost section of the enteric canal 

 remains permanently bent downwards and is to be designated as the 

 anal gut. 



The assertion is, therefore, justified that the growth of the enteroderm, 

 i.e. of the enteric canal, which has been delayed ever since the formation 

 of the blastula (see Hatta, 07), is accelerated at about the fourteenth day. 

 It follows that the more than twenty mesodermic somites which had been 

 added in the course of the previous ten days (from the fourth to the four- 

 teenth day), and the prolongation of the chorda, which took place in the 

 same interval of time, are not caudal, though they appear to be the product 

 of the tail bud, but belong from the first to the trunk. The mesodermic 

 somites and the chorda belonging to the tail are formed only after this 

 period. The account given by Goette (90) that the caudal somites are 

 shifted into the trunk, is accordingly not correct ; for they are not yet 

 formed at the stage spoken of. 



I mention in particular this fact, because between the mesodermic 

 somites in the trunk and those in the tail there is at this stage a marked 

 contrast : in the former series the pronephrotomes are converted either 

 into the pronephric tubules or into the segmental duct (see Hatta, 00), 

 whereas in thelatter series, which appears later, this is not the case. 



By the fourteenth day the tail appears as an external growth. The 

 medullary cord which comes first under consideration, is cut off already in 

 the course of the thirteenth day from the epidermal ectoderm which is 



