60 



EMBRYOLOGY 



frothy-looking endoplasm can be distinguished in them. 

 Cleavage is always total and equal, and leads first to a 

 moruJa-stage, which shows no cleavage cavity within it. By 

 the development of a layer of small ciliated cells on its 

 outer surface, there is produced a two-layer spherical or 

 somewhat elongated planula-stage. Nothing moi'e accurate 

 concerning the separation of the two germ-layers is known 

 up to the present time. 



The development of the Siphonophora has been investi- 

 gated chiefly by Gegenbaur (No. 67), Haeckbl (Nos. 68 and 

 70), Metschnikoff (No. 13), Fewkes (No. 66), and Chun 

 (Nos. 54 — 58). Considerable differences prevail among the 

 various groups regarding the further development (meta- 

 morphosis^) of the young Siphonophore stock. 



Physophoridae. — A comparatively simple type is repre- 

 sented by the development of Halistemma (Stephanomia) 

 pictum. The first change noticeable in the planula is an 

 elongation in the direction of the subsequent chief axis 

 (Fig. 24 A) and the accumulation of red pigment at the 

 lower (oral) pole. Certain small cells, Avhich have ap- 

 parently proceeded from a metamorphosis of the large 

 nutritive entoderm cells, then make their appearance under 

 the ectodermal cell-layer, and soon arrange themselves in a 

 second layer of cells (the permanent entoderm) under the 

 ectoderm. In the further course of development the large 

 nutritive entodermal elements become more and more ab- 

 sorbed, so that an internal cavity, the gastrovascular cavity, 

 is developed (Fig. 24 B). The fundament of the first 

 organ to be formed is seen at the upper (aboral) pole. The 

 ectoderm here exhibits a thickening, which very soon, like 

 the bud nucleus l_Knospenker7i'\ of a medusa, grows inward 

 (Fig. 24 A, ep), and by a dehiscence of the cells develops 



* We here regard the entire Siphonojihore stock as a unit (individual 

 of the third or higher degree, co?-m). Just as the metamoriDhosis of an 

 individual of the second degree (person) usually takes place by the loss 

 of larval organs and their replacement by permanent ones, so the meta- 

 morphosis of the Sii^honophore stock is frequently accompanied by the 

 loss of larval parts, to which the value of a person must be ascribed, e.g., 

 nectocalyces, hydrophyllia, etc. 



