58 PROFESSOR E. RAY LANKESTER. 



consideration) that the Archigastrula is always what I have 

 termed an * invaginate Gastrula or Planula/ and, of course, 

 since the cells are of equal size, and the invagination is a real 

 pushing-in, it corresponds with my ' embolic invaginate ' 

 type ; the ' epibolic invaginate ' type, where the smaller cells 

 of the blastosphere grow over the larger cells being classed 

 under the Amphiblastic type. Haeckel is inclined to doubt 

 whether such a thing as a ' Delaminate Gastrula or Planula ' 

 ever is formed ; that is to say, he does not think that the observa- 

 tions are satisfactory in which the endodermal layer of cells 

 have been stated to arise by cell- division on the inner surface of 

 the blastosphere.^ Very probably careful observations would 

 show in such cases an invagination at a very early period. We 

 are in want of observations (by means of section-cutting) on the 

 development of the Hydroid polyps. But even if delamina- 

 tion does occur it can only be regarded as a heterochrony. 

 The fate of the ^ Urmund,^ as Haeckel calls it, the orifice of 

 invagination or ' blastopore/ as I prefer to say, is not 

 discussed by him. It is one of the points upon which the 

 whole superstructure of the Gastreea theory rests, as I have 

 pointed out (this Journal, April, 1875). Haeckel, however, 

 insists on the importance of recognising the rim or margin 

 of the blastopore, which he calls Properistoma. It is, he 

 holds, identical with the much discussed and highly im- 

 portant ' Randwulst ' or ' Keimwulst ' of the discoblastic 

 type of development (Chick, Shark, Cuttle-fish). It is here, 

 in the annular space between Entoderm and Exoderm, that 

 the first cells are separated from the two primary germ-layers 

 to give rise to the mesoderm, or mesoblast. 



The Amphiblastic type{V\. X). — The most familiar instance 

 of this type of development, which consists essentially in the 

 unequal size of the cleavage-cells, a portion of Avhich are large 

 and charged with food-material, is seen in the frog and other 

 Amphibia, also in Petromyzon and Accipenser. Probably 

 the Dipneusta and placental Mammals belong to this category. 

 The greater number of the MoUusca are amphiblastic, 

 probably even some Cephalopods (the commoner forms of 

 which are discoblastic) and some Brachiopods (others being 

 archiblastic) . Among the Arthropoda amphiblastic develop- 

 ment appears to occur only in some of the lower Crustacea 

 and Tracheata. Amongst the Echinodermata only certain 

 Asterida and Holothurida, with a so-called 'direct' or ^abbre- 

 viated ' development, present the amphiblastic type. A large 

 majority of the Worms are to be reckoned here, and amongst 



^ It does not seem possible to explain Fol's observations on the develop- 

 ment of Geryonidse as indicating anything but a true ' delamination.' 



