Soiui- Plienomciui of J^i-gciKTatiuii in Lininudrilu.s and irlatcd Formn. 417 



cause of the abnorniality is not eiitirely clear. In searching for an 

 explanation one naturally thinks of an injury to the nerve cord but 

 the condition of this was sueli tliat under normal circumstances one 

 would not suspect an injury and the probabilities are against it since 

 the nerve was some distance from the opening and directly opposite 

 the latter it seemed normal. Neither did the nerve come in contact 

 with the intestine so that any share which a nervous Stimulus may have 

 had in causing the abnonnality must have been entirely secondary. In 

 the ectoderm at the point of contact with the intestine the invagi- 

 nation is not deep, the thickening is not great and a migration of the 

 ectoderm into the coelom occurs at but one point and even here it 

 is slight. It seems probably, therefore, that this abnormal condition 

 (if the ectoderm occured after the intestine had touched it. That the 

 presence of the intestine at a cut surface of the body wall other than 

 at the posterior end of the body may cause the formation of an anal 

 opening was noticed by Hirschler. Upon cutting off the posterior 

 end of a leech rather obliquely so as to sever not only the posterior 

 portion of the intestine but also one of the posteriorly directed lateral 

 caeca he found that in addition to the normal anus a small lateral anus 

 had likewise formed opposite the injured caecum. In the present 

 instance the question centers about the factor that caused the intestine 

 to grow over toward the body wall. It is possible that the intestine 

 had been pierced by the needle used to hold the worm or the body 

 musculature might have been injured at this point and formed a Strand 

 which extended over and deflected the intestine. Such Strands in other 

 individuals had partially deflected the tip of the growing intestine 

 but the specimens had been killed before it could be determined what 

 the final result would have been. The musculature of the wall for a 

 Short distance on either side of the Ups is disorganized and it seems 

 probable that a Strand may have at least directed the groAvth of the 

 intestine to one side. 



The Ectoderm and the Neoblasts and Formation of the 

 Mesoderm. 



Although under the abnormal conditions that we are considering 

 the body wall does not elongate, still it is not in what might be termed 

 a quiescent State. The ectoderm cells about the wound are actively 

 dividing and as in normal regulation the proliferatien is abnost entir- 

 ely confined to the ventral half of the body. The process is initiated 

 by a marked enlargement of the cells after which they divide both 



27* 



