haeckel's recent additions to gastr^a-theory. 63 



Van Beneden, may not be largely diifused. In this case the 

 separation of food-material from formative protoplasm does 

 not take place at an early period, nor so as to produce a 

 superficial homogeneous layer of protoplasm, covering-in a 

 granular yelk. But the segregation takes place at separate 

 points, so that a number of isolated cells (exactly comparable 

 in origin to the Cephalopod's autoplasts) rise to the surface of 

 the yelk and then proceed to divide, and so form a complete 

 Perimorula. Now, it is not possible to say (in the absence 

 of observations) how those cells arose, but it seems probable, 

 from the analogy of the Cephalopods, that there was not a 

 primitive division into two, and then into four, and so on, 

 which took place deeply in amongst the granular mass of food- 

 material, but, on the contrary, it would appear most likely 

 that the network of protoplasm spread in and out amongst 

 the food-granules segregated simultaneously at many points, 

 and gave rise to autoplasts. Just as, in the Cephalopod, the 

 segregation to form the cleavage-disc is incomplete, and leaves 

 material behind which independently segregates at many 

 points as autoplasts — so in Gammarus jiuviatilis , and probably 

 other periblastic species, the segregation to form the super- 

 ficial cleavage-tunic or rind never comes off, but is reduced 

 to a simultaneous formation of many autoplasts. In such 

 cases the fertilised e^^ passes at once to tbe Periblastula stage, 

 and cannot be said to exhibit either a Pm-monerula or Peri- 

 cytula, or Peri-morula stage. 



The food-material in the cells of the Perimorula is so placed 

 as to occupy the inner face of each cell, and so lie centrally, 

 but it is still disposed in the substance of the cells. By the 

 gradual separation of the cells from it, it comes to occupy a 

 central cavity, and is now surrounded by the cells which have 

 freed themselves from it. This is the stage of the Periblas- 

 tula (Fig. 86). 



The invagination of the Periblastula to form the Perigas- 

 trula, a fundamentallyi mportant fact for the whole of the 

 germ-layer theory, was first observed by Bobretzky in Astacus 

 and Palsemon in 1873. It is exceedingly curious to find that 

 in the periblastic type the invagination is so well marked, 

 and the observation lends a certain probability to the 

 hypothesis that invagination is the primary mode of forma- 

 tion of the Gastrula. 



It is also exceedingly important and interesting to find that 

 here, as elsewhere, the orifice of invagination or blastopore 

 (Urmund of Haeckel) does not become the animal's mouth. 

 Haeckel is not able to say, in the case of Peneus, whether it 

 becomes the anus or not (see Plate IX, and its explanation). 



