HAECKEL S RECENT ADDfTIONS TO GASTR^A-THEORY. 55 



the developing organism belongs. Thus in the Vertebrata 

 the iiotochord and the branchial slits are out of all proportion 

 early in their appearance, and large in relation to their adult 

 size and the size of other organs ; whilst, on the other hand, 

 those organs are more and more pushed into the background 

 ■which have the most general significance for all Mctazoa. 

 Hence above all other organs we find the primitive alimentary 

 canal,^ or archenteron, and the primitive mouth - suffering 

 in this respect in so far as their primitive form is concerned. 

 Hence too the simple, primordial Gastrula, which arises by 

 the invagination of a perfectly simple sac, the wall of which 

 consists of a single layer of cells (Blastula), is especially 

 preserved in the most faithful Avay by the lowest, most indif- 

 ferent, and oldest forms of the different groups (for example, 

 in the ontogeny of Gastrophysema, Monoxenia, Sagitta, 

 Phoronis, Argiope, Terebratula, Uraster, Toxopneustes, 

 Ascidia, Amphioxus).3 



Palingeneiic plastic yelk and cenogenetic food-material. 

 — The great importance of distinguishing these two elements 

 and their share in early developmental phenomena is insisted 

 upon by Haeckel. He holds that (as I have maintained in 

 relation to the developmental phenomena of the Mollusca) 

 the food-material thrown in to the primitive egg-cell dis- 

 turbs and clouds the subsequent development in the most 

 profound manner, hindering and concealing the full unfolding 

 of the palingenesis. In all cases Haeckel maintains that the 

 e^^ is palingenetically a single cell and that the food-material 

 is a cenogenetic addition or adaptation.* 



^ Since we have no words equivalent to the German Darm and Urdarm, 

 we may use the Greek equivalents enteron and archenteron. 



" Or ' blastopore.' 



^ The examples cited by Haeckel do not appear to me altogether to 

 warrant this conclusion. Sagitta, Terebratula, and Toxopneustes are not 

 generally admitted to be either indifferent or archaic types of the groupsto 

 which they belong. And even if they were it is not easy to follow out the 

 reasoning by which it is concluded that they should therefore be expected 

 to exhibit the simplest form of gastrula, for although assumed to be low in 

 their respective groups, yet there is a very long distance between any of 

 them (except the Sponges) and the ancestral gastrula, a distance sufficient 

 to allow of the development of countless heterotopisms and hetero- 

 chronisms, and without doubt sufficient to allow of the development of that 

 particular cenogenetic phenomenon which beyond all others is efficient in 

 obscuring the gastrula form, viz. the addition to the egg-cell of food-yelk, 

 deutoplasm, or ' food-material.' I pointed out the important influence of 

 this accessory yelk in my original paper on this subject, ' Ann. Nat. Hist.,' 

 May, 1873, p. 328, and in subsequent embryological writings. 



* From one point of view the admixture of food-material with the proto- 

 plasmic egg-cell may be regarded as a palingenetic survival of the ' amoeboid 

 form of nutrition,' in which solid food is taken bodily into the living sub- 



