File:Frazer1928 fig04.jpg

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Text-fig. 4. Schematic analysis of 21 mm stage

(left), with section through isthmus (right).

The growth of the alar lamina, which has led to the changes on the outer aspect which have just been considered, has its effect also on the internal appearance, as has been shown in Plate I. As already mentioned, however, this appearance is deceptive. The schematic analysis of the part is given in text-fig. 4, where it can be seen that the alar region, although carried forward and separated by what is apparently an interlaminar sulcus from the (lower part of the) basal lamina, is occupying here a situation in which the marking of the basal structure is still represented. The explanation is that the basal lamina remains, and alone forms the side wall of the cavity here, while the derivatives of the alar lamina, in growing forward, pass on its outer side. The appearance of an interlaminar sulcus is produced by a modification of a condition faintly foreshadowed in the 16 mm embryo.

In this way, then, the section across the isthmus (text-fig. 4) of the 21 mm. specimen shows the basal lamina lining the cavity, with the deep translaminar sulcus cutting into it, while a small ependymal thickening, low down, marks all that is left of the originally deep intralaminar groove. The association fibres (or posterior longitudinal bundle, as one might now venture to term the collection) extend now well above the intralaminar remnant, and the lamina is thickening outside this, with a scattered mantle layer of nuclei. The alar lamina is now represented by a. small area of the section outside and above the translaminar sulcus. This addition to the section has included the issuing nerve in part, burying it in its course to a small extent.

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Frazer JE. Development of the region of the isthmus rhombencephali. (1928) J Anat. 63: 7-18. PMID 17104212

Cite this page: Hill, M.A. (2024, February 28) Embryology Frazer1928 fig04.jpg. Retrieved from

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