246 SUMMARY OF CURRENT RESEARCHES RELATING TO 



blastomcrcs divide into four smaller and aboral primitive ectoderm- 

 colls, and into four larger and oral primitive endodcrm-cells ; some 

 of tlio latter are used up as nutrient yolk ; the enteron is at first 

 solid (except, according to Goettc, in Stiilochtis pilidium'). There is 

 epiboly, and the blastopore lies at about the middle of the future 

 ventral side. Whether it remains open or closes, it always marks the 

 point where the stomodn>nm is afterwards formed from the ectoderm. 

 The aboral pole, and all the organs that arise in its neighbourhood, 

 gradually migrate from the middle of the dorsal side towards the 

 anterior end of the body, so that the oral orifice is pushed more and 

 more backwards. 



The ectoderm is never multilaminate ; the rhabdites arise as 

 concretions in its cells. The first two or three eyes arise in the ecto- 

 derm and thence pass into the mesoderm. All the other eyes arise 

 in the mesoderm, but by division of the first two or three. The 

 sensory and the motor portions of the brain probably arise from the 

 ectoderm independently of one another ; the latter appears at the 

 aboral pole below or on either side of what will be the anterior 

 median branch, of the enteron ; the former is at first isolated at the 

 aboral pole, and only secondarily passes on either side of the anterior 

 median branch to unite with the ventral sensory portion. The primary 

 nerve-trunks arise from the brain. 



The mesoderm, formed by the repeated division of the four 

 primitive mesoderm-cells, never becomes distinctly divided into a 

 dermo-fibrous and an entero-fibrous layer ; the parenchyma and all 

 the muscular elements of the body are derived from it. 



In all the forms examined the central cavity of the enteron was 

 found to be formed by the resorption of the central yolk-masses of 

 the enteroderm, the enteric cells passing to the periphery and there 

 forming an epithelium. 



All the P. cotylea and all the P. acotylea, except the Leptoplanidse, 

 pass through a metamorphosis ; and the metabolic forms leave the 

 egg-shell luucb earlier than those which dcvelope directly ; they have 

 a common larval form — the Muellerian larva — which is characterized 

 externally by a pre-oral ciliated band, which is drawn out on eight 

 processes. The mode of arrangement and movement of the cilia 

 call to mind the ctenophores of the Ctenophora. All the metabolic 

 forms have the Muellerian larva preceded by that of Goette, in which 

 there are four processes ; the only larval organs are the ciliated band 

 and the processes ; when these are absorbed, no portion of the larva 

 is cast ofi". There are many facts which go to show that the develop- 

 ment with metamorphosis recapitulates more primitive arrangements 

 than does the direct mode of development. Of the metabolic Poly- 

 clads, those again seem to be the earliest whose embryonic life is 

 shortest. 



The neck-tentacles and tbe marginal tentacles are, from the first, 

 dificrent structures. The former are solid processes of the body, the 

 latter thickenings and foldings of the most anterior edge of the body. 

 There is no true anus, and the ectodermal spaces by which a small 

 space is loft cannot be regarded at most as being more than the 



